kvm-unit-tests: VMX: Test suite for preemption timer

Test cases for preemption timer in nested VMX. Two aspects are tested:
1. Save preemption timer on VMEXIT if relevant bit set in EXIT_CONTROL
2. Test a relevant bug of KVM. The bug will not save preemption timer
value if exit L2->L0 for some reason and enter L0->L2. Thus preemption
timer will never trigger if the value is large enough.
3. Some other aspects are tested, e.g. preempt without save, preempt
when value is 0.

Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>