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>