qemu.tests: add check clock offset when vm crash/bsod

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: add check clock offset when guest bsod/crash

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: new case to check clock offset after hotplug vcpus

check guest clock drift less that $tolerance after online/offline
hotpluged vcpus;

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: cfg for checking clock offset after hotplug vcpu

Signed-off-by: Xu Tian <xutian@redhat.com>