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: NMax Reitz <mreitz@redhat.com> Reviewed-by: NKevin Wolf <kwolf@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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