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