iotests: Only kill NBD server if it runs

iotests: Only kill NBD server if it runs

There may be NBD tests which do not create a sample image and simply
test whether wrong usage of the protocol is rejected as expected. In
this case, there will be no NBD server and trying to kill it during
clean-up will fail.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit f798068c565918ead63218d083ff814b7635be72)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>