get_msr() of MSR_EFER is currently always going to succeed, but static
checker doesn't see that far.

Don't complicate stuff and just use 0 for the fallback -- it means that
the feature is not present.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>