KVM: x86: Add helper to get variable MTRR range from MSR index

Add a helper to dedup the logic for retrieving a variable MTRR range
structure given a variable MTRR MSR index.

No functional change intended.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230511233351.635053-5-seanjc@google.com


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