migration: use qemu-sockets to establish Unix sockets

This makes migration-unix.c again a cut-and-paste job from migration-tcp.c,
exactly as it was in the beginning. :)

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
migration (outgoing): add error propagation for all protocols

Error propagation is already there for socket backends, but it
is (and remains) incomplete because no Error is passed to the
NonBlockingConnectHandler.

With all protocols understanding Error, the code can be simplified
by removing the return value.

Before:

    (qemu) migrate fd:ffff
    migrate: An undefined error has occurred
    (qemu) info migrate
    (qemu)

After:

    (qemu) migrate fd:ffff
    migrate: File descriptor named 'ffff' has not been found
    (qemu) info migrate
    capabilities: xbzrle: off
    Migration status: failed
    total time: 0 milliseconds

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
migration (incoming): add error propagation to fd and exec protocols

And remove the superfluous integer return value.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-char: ask and print error information from qemu-sockets

Before:

    $ qemu-system-x86_64 -monitor tcp:localhost:6000
    (starts despite error)

    $ qemu-system-x86_64 -monitor tcp:foo.bar:12345
    getaddrinfo(foo.bar,12345): Name or service not known
    chardev: opening backend "socket" failed

    $ qemu-system-x86_64 -monitor tcp:localhost:443,server=on
    inet_listen_opts: bind(ipv4,127.0.0.1,443): Permission denied
    inet_listen_opts: FAILED
    chardev: opening backend "socket" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -monitor tcp:localhost:6000
    x86_64-softmmu/qemu-system-x86_64: -monitor tcp:localhost:6000: Failed to connect to socket: Connection refused
    chardev: opening backend "socket" failed

    $ x86_64-softmmu/qemu-system-x86_64 -monitor tcp:foo.bar:12345
    qemu-system-x86_64: -monitor tcp:foo.bar:12345: address resolution failed for foo.bar:12345: Name or service not known
    chardev: opening backend "socket" failed

    $ x86_64-softmmu/qemu-system-x86_64 -monitor tcp:localhost:443,server=on
    qemu-system-x86_64: -monitor tcp:localhost:443,server=on: Failed to bind socket: Permission denied
    chardev: opening backend "socket" failed

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
nbd: ask and print error information from qemu-sockets

Before:

    $ qemu-system-x86_64 nbd:localhost:12345
    inet_connect_opts: connect(ipv4,yakj.usersys.redhat.com,127.0.0.1,12345): Connection refused
    qemu-system-x86_64: could not open disk image nbd:localhost:12345: Connection refused

After:

    $ x86_64-softmmu/qemu-system-x86_64 nbd:localhost:12345
    qemu-system-x86_64: Failed to connect to socket: Connection refused
    qemu-system-x86_64: could not open disk image nbd:localhost:12345: Connection refused

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-ga: ask and print error information from qemu-sockets

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
vnc: add error propagation to vnc_display_open

Before:

    $ qemu-system-x86_64 -vnc foo.bar:12345
    getaddrinfo(foo.bar,18245): Name or service not known
    Failed to start VNC server on `foo.bar:12345'

    $ qemu-system-x86_64 -vnc localhost:12345,reverse=on
    inet_connect_opts: connect(ipv4,yakj.usersys.redhat.com,127.0.0.1,12345): Connection refused
    Failed to start VNC server on `localhost:12345,reverse=on'

After:

    $ x86_64-softmmu/qemu-system-x86_64 -vnc foo.bar:12345
    qemu-system-x86_64: address resolution failed for foo.bar:18245: Name or service not known
    Failed to start VNC server on `foo.bar:12345'

    $ x86_64-softmmu/qemu-system-x86_64 -vnc localhost:12345,reverse=on
    qemu-system-x86_64: Failed to connect to socket: Connection refused
    Failed to start VNC server on `localhost:12345,reverse=on'

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-sockets: include strerror or gai_strerror output in error messages

Among others, before:

    $ qemu-system-x86_64 -chardev socket,port=12345,id=char
    inet_connect: host and/or port not specified
    chardev: opening backend "socket" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -chardev socket,port=12345,id=char
    qemu-system-x86_64: -chardev socket,port=12345,id=char: host and/or port not specified
    chardev: opening backend "socket" failed

perror and fprintf can be removed because all clients can now
consume Errors properly.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-sockets: add error propagation to inet_connect_addr

perror and fprintf can be removed because all clients can now consume
Errors properly.  However, we need to change the non-blocking connect
handlers to take an Error, in order to improve error handling for
migration with the TCP protocol.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-sockets: add error propagation to inet_dgram_opts

Before:

    $ qemu-system-x86_64 -monitor udp:localhost:631@localhost:631
    inet_dgram_opts: bind(ipv4,127.0.0.1,631): OK
    inet_dgram_opts failed
    chardev: opening backend "udp" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -monitor udp:localhost:631@localhost:631
    qemu-system-x86_64: -monitor udp:localhost:631@localhost:631: Failed to bind socket: Address already in use
    chardev: opening backend "udp" failed

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-sockets: add error propagation to inet_parse

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-sockets: add error propagation to Unix socket functions

Before:

    $ qemu-system-x86_64 -monitor unix:/vvv,server=off
    connect(unix:/vvv): No such file or directory
    chardev: opening backend "socket" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -monitor unix:/vvv,server=off
    qemu-system-x86_64: -monitor unix:/vvv,server=off: Failed to connect to socket: No such file or directory
    chardev: opening backend "socket" failed

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>