The Intel ISR section for RDMSR seems to say: "Specifying a reserved
or unimplemented
MSR address in ECX will also cause a general protection exception".
From a guest's perspective, MSR_RAPL_POWER_UNIT is unimplemented; kvm matches
this behavior.
MSRs are model specific and defined per model number. If you report a model
number you're expected to implement the MSRs defined for that model number.
AFAIK Xen just reports 0 for unknown MSRs (and I'm surprised KVM doesn't too)
I would suggest to fix KVM.