qemu-iotests/162: Fix leaked temporary files

qemu-iotests 162 left qemu-nbd.pid behind in the scratch directory, and
potentially a file called '42' in the current directory. Make sure to
clean it up after completing the tests.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
