KVM: x86: Add helper to query if variable MTRR MSR is base (versus mask)

Add a helper to query whether a variable MTRR MSR is a base versus as mask
MSR.  Replace the unnecessarily complex math with a simple check on bit 0;
base MSRs are even, mask MSRs are odd.

Link: https://lore.kernel.org/r/20230511233351.635053-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>