nbd: close socket if connection breaks

nbd_receive_reply() is called by the event loop whenever data is
available or the socket has been closed by the remote side.

This patch closes the socket when an error occurs to prevent the
nbd_receive_reply() handler from being called indefinitely after the
connection has failed.

Note that we were already correctly returning EIO for pending requests
but leaving the nbd_receive_reply() handler registered resulted in high
CPU consumption and a flood of error messages.

Reuse nbd_teardown_connection() to close the socket.

Reported-by: Zhifeng Cai <bluewindow@h3c.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
tests: add nbd-fault-injector.py utility

The nbd-fault-injector.py script is a special kind of NBD server.  It
throws away all writes and produces zeroes for reads.  Given a list of
fault injection rules, it can simulate NBD protocol errors and is useful
for testing NBD client error handling code paths.

See the patch for documentation.  This scripts is modelled after Kevin
Wolf <kwolf@redhat.com>'s blkdebug block driver.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
qemu-iotests: add 083 NBD client disconnect tests

This new test case uses nbd-fault-injector.py to simulate broken TCP
connections at each stage in the NBD protocol.  This way we can exercise
block/nbd-client.c's socket error handling code paths.

In particular, this serves as a regression test to make sure
nbd-client.c doesn't cause an infinite loop by leaving its
nbd_receive_reply() fd handler registered after the connection has been
closed.  This bug was fixed in an earlier patch.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
qemu-iotests: remove 085 and 087 from 'quick' group

The 'quick' group in qemu-iotests are not allowed to run QEMU since we
don't know which targets are available.  In other words, they may only
use qemu-img, qemu-io, and qemu-nbd.

Drop 085 and 087 from the 'quick' group since they run QEMU.  This
makes "make check-block" pass again.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>