libvirt.tests: add new case into virsh send-key command

$ ./run -t libvirt --test type_specific.io-github-autotest-libvirt.virsh.sendkey.sysrq.non_acl.reboot_guest --no-downloads --keep-image --keep-image-between-tests
SETUP: PASS (0.56 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /tmp/virt-test/logs/run-2014-07-16-15.06.11/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.sendkey.sysrq.non_acl.reboot_guest: PASS (45.68 s)
TOTAL TIME: 45.74 s
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %

$ ./run -t libvirt --test type_specific.io-github-autotest-libvirt.virsh.sendkey.sysrq.acl_test.reboot_guest --no-downloads --keep-image --keep-image-between-tests
SETUP: PASS (0.57 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /tmp/virt-test/logs/run-2014-07-16-15.04.50/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.sendkey.sysrq.acl_test.reboot_guest: PASS (64.16 s)
TOTAL TIME: 64.22 s (01:04)
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %
So what's the clien_session used for?