KVM: MTRR: handle MSR_MTRRcap in kvm_mtrr_get_msr

MSR_MTRRcap is a MTRR msr so move the handler to the common place, also
add some comments to make the hard code more readable

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>