libvirt.tests: add new test case for guest kernel debugging feature

$ ./run --selinux-setup -t libvirt --test type_specific.io-github-autotest-libvirt.virsh.guest_kernel_debugging.positive_testing.send_nmi --no-downloads --keep-image
SETUP: PASS (0.74 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /tmp/virt-test/logs/run-2014-07-16-11.37.34/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.guest_kernel_debugging.positive_testing.send_nmi: PASS (70.11 s)
TOTAL TIME: 70.17 s (01:10)
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %

This PR is based on https://github.com/autotest/virt-test/pull/1715
Yes, it's not necessary, we have test configurations in parent cfg.