Introduce accessors to retrieve feature bits from CPUID entries and use
the new accessors where applicable.  Using the accessors eliminates the
need to manually specify the register to be queried at no extra cost
(binary output is identical) and will allow adding runtime consistency
checks on the function and index in a future patch.

No functional change intended.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>