Add test cases for getting and setting secret value of the volume

Test cases covered basic secret operations and focus on getting and setting secret value of the volume, the following is test report, please help review, thanks.

# ./run -t libvirt --tests virsh_secret_set_get --no-downloads
SETUP: PASS (0.30 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /home/ajia/Workspace/github/virt-test/logs/run-2013-10-12-16.12.41/debug.log
TESTS: 10
(1/10) virsh_secret_set_get.create_secret_volume.secret_private: PASS (0.66 s)
(2/10) virsh_secret_set_get.create_secret_volume.secret_public: PASS (0.35 s)
(3/10) virsh_secret_set_get.negative_testing.get_secret_value.no_secret_value: PASS (0.45 s)
(4/10) virsh_secret_set_get.negative_testing.get_secret_value.invalid_options: PASS (0.34 s)
(5/10) virsh_secret_set_get.negative_testing.set_secret_value.noexist_secret_uuid: PASS (0.34 s)
(6/10) virsh_secret_set_get.negative_testing.set_secret_value.invalid_options: PASS (0.27 s)
(7/10) virsh_secret_set_get.positive_testing.set_secret_value.secret_public: PASS (0.29 s)
(8/10) virsh_secret_set_get.positive_testing.get_secret_value.secret_public: PASS (0.32 s)
(9/10) virsh_secret_set_get.positive_testing.set_secret_value.secret_private: PASS (0.28 s)
(10/10) virsh_secret_set_get.positive_testing.get_secret_value.secret_private: PASS (0.32 s)
TOTAL TIME: 3.66 s
TESTS PASSED: 10
TESTS FAILED: 0
SUCCESS RATE: 100.00 %