qemu-iotests: improve nbd-fault-injector.py startup protocol

qemu-iotests: improve nbd-fault-injector.py startup protocol

Currently 083 waits for the nbd-fault-injector.py server to start up by
looping until netstat shows the TCP listen socket.

The startup protocol can be simplified by passing a 0 port number to
nbd-fault-injector.py.  The kernel will allocate a port in bind(2) and
the final port number can be printed by nbd-fault-injector.py.

This should make it slightly nicer and less TCP-specific to wait for
server startup.  This patch changes nbd-fault-injector.py, the next one
will rewrite server startup in 083.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20170829122745.14309-3-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>