KVM: x86: move MSR_IA32_POWER_CTL handling to common code

Make it available to AMD hosts as well, just in case someone is trying
to use an Intel processor's CPUID setup.

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>