qemu.tests: new case to check guest clock offset

Test cases: 
1. Check vm clock offset with syscall;
2. Check vm clock offset when guest crash or BSOD;
3. Check vm clock offset after online/offline plugged  vcpus;
4. Check kvm-clock using newer msr set;
Signed-off-by: Xu Tian <xutian@redhat.com>