KVM: x86: Split out logic to generate "readable" APIC regs mask to helper

Move the generation of the readable APIC regs bitmask to a standalone
helper so that VMX can use the mask for its MSR interception bitmaps.

No functional change intended.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20230107011025.565472-5-seanjc@google.com


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