libvirt.test: Add some more normal/error cases about sync test

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
After change session.cmd to session.cmd_output in line 44, run the test got fail as:
```
11:13:04 INFO | Original dirty data: 2336
11:13:04 INFO | Frozen dirty data: 4
11:13:04 INFO | Thawed dirty data: 104
11:13:04 DEBUG| Sending command: rm -f /tmp/test.file
11:13:05 DEBUG| Sending command: echo $?
11:13:05 DEBUG| Undefine VM virt-tests-vm1
11:13:05 DEBUG| Define VM from /tmp/xml_utils_temp_QZIrbj.xml
11:13:06 WARNI| Requested MAC address release from persistent vm virt-tests-vm1. Ignoring.
11:13:06 DEBUG| Checking image file /home/wayne/work/virt-test/shared/data/images/jeos-19-64.qcow2
11:13:06 DEBUG| Running '/bin/qemu-img info /home/wayne/work/virt-test/shared/data/images/jeos-19-64.qcow2'
11:13:06 DEBUG| Running '/bin/qemu-img check /home/wayne/work/virt-test/shared/data/images/jeos-19-64.qcow2'
11:13:06 DEBUG| Copying /home/wayne/work/virt-test/shared/data/images/jeos-19-64.qcow2.backup -> /home/wayne/work/virt-test/shared/data/images/jeos-19-64.qcow2
11:13:07 DEBUG| (address cache) DHCP lease OK: aa:18:df:fd:72:38 --> 10.66.84.36
11:13:14 DEBUG| Encoding video file /home/wayne/work/virt-test/logs/run-2014-07-25-11.12.32/type_specific.io-github-autotest-libvirt.virsh.qemu_agent_command_fs/screendumps_virt-tests-vm1_8110-0.ogg
11:13:14 INFO | Video creation failed for /home/wayne/work/virt-test/logs/run-2014-07-25-11.12.32/type_specific.io-github-autotest-libvirt.virsh.qemu_agent_command_fs/screendumps_virt-tests-vm1_8110: multifilesrc
11:13:15 DEBUG| Running 'true'
11:13:15 DEBUG| Running 'ps -o comm 1'
11:13:15 ERROR| 
11:13:15 ERROR| Traceback (most recent call last):
11:13:15 ERROR|   File "/home/wayne/work/virt-test/virttest/standalone_test.py", line 219, in run_once
11:13:15 ERROR|     run_func(self, params, env)
11:13:15 ERROR|   File "/home/wayne/work/virt-test/test-providers.d/downloads/io-github-autotest-libvirt/libvirt/tests/src/virsh_cmd/domain/virsh_qemu_agent_command_fs.py", line 85, in run
11:13:15 ERROR|     raise error.TestFail("The frezon dirty data should be 0!")
11:13:15 ERROR| TestFail: The frezon dirty data should be 0!
11:13:15 ERROR| 
11:13:15 ERROR| FAIL type_specific.io-github-autotest-libvirt.virsh.qemu_agent_command_fs -> TestFail: The frezon dirty data should be 0!
```

The strict check "fz_dirty_info" equal 0 fail here, don't know why, any idea?