qemu: Add more test cases for timer devices

Tested in Fedora rawhide host, with guests:
Fedora 19 
```
18:28:53 INFO | Job total elapsed time: 1792.65 s
18:28:53 INFO | Tests passed: 20
18:28:53 INFO | Tests failed: 0
18:28:53 INFO | Success rate: 100.00 %
```
Win7:
```
19:24:08 INFO | Job total elapsed time: 2196.78 s
19:24:08 INFO | Tests passed: 20
19:24:08 INFO | Tests failed: 0
19:24:08 INFO | Success rate: 100.00 %
```
RHEL6.4:
```
00:46:35 INFO | Job total elapsed time: 5396.69 s
00:46:35 INFO | Tests passed: 40
00:46:35 INFO | Tests failed: 2
00:46:35 INFO | Success rate: 95.24 %
```
The 2 failed tests is `clock_drift_with_sleep` and `change_guest_clksource`, for the guest has larger time drift than expected, I need to confirm with developer about this result.