KVM: VMX: Move MSR_IA32_FEAT_CTL.LOCKED check into "is valid" helper

Move the check on IA32_FEATURE_CONTROL being locked, i.e. read-only from
the guest, into the helper to check the overall validity of the incoming
value.  Opportunistically rename the helper to make it clear that it
returns a bool.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220607232353.3375324-3-seanjc@google.com