Correct rdtsc() with the KVM steal clock

[MatiasVara]

Currently, we are using the rdtsc() register to measure cycles but the register is not virtualized so the measure is not always valid since the vcpu may have been stole. To use the rdtsc register, we need to take into account the kvm steal clock and correct the value of rdtsc.

[MatiasVara]

See https://github.com/torvalds/linux/blob/610a9b8f49fbcf1100716370d3b5f6f884a2835a/arch/x86/kernel/kvm.c#L328