qemu-file: drop socket_put_buffer

It is enough to implement one of socket_writev_buffer and
socket_put_buffer.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iov: reorganize iov_send_recv, part 1

Once the initial part of the iov is dropped, it is not used anymore.
Modify iov/iovcnt directly instead of adjusting them with the "si"
variable.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iov: reorganize iov_send_recv, part 2

Do not touch the "bytes" argument anymore.  Instead, remember the
original length of the last iovec if we touch it, and restore it
afterwards.

This requires undoing the changes in opposite order.  The previous
algorithm didn't care.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iov: reorganize iov_send_recv, part 3

"si" and "ei" are merged in a single variable.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iov: handle partial writes from sendmsg and recvmsg

Partial writes can still happen in sendmsg and recvmsg, if a
signal is received in the middle of a write.  To handle this,
retry the operation with a new offset/bytes pair.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-file: do not use stdio for qemu_fdopen

This uses system calls directly for Unix file descriptors, so that the
efficient writev_buffer can be used.  Pay attention to the possibility
of partial writes in writev.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>