x86: Add RDTSC test

Verify that the difference between a guest RDTSC instruction and the
IA32_TIME_STAMP_COUNTER MSR value stored in the VMCS12's VM-exit
MSR-store list is less than 750 cycles, 99.9% of the time.

Testcase for Linux kernel commit 662f1d1d1931 ("KVM: nVMX: Add support
for capturing highest observable L2 TSC”).

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>