xen-detect: make CPUID fallback CPUID-faulting aware

xen-detect: make CPUID fallback CPUID-faulting aware

Relying on presence / absence of hypervisor leaves in raw / escaped
CPUID output cannot be used to tell apart PV and HVM on CPUID faulting
capable hardware. Utilize a PV-only feature flag to avoid false positive
HVM detection.

While at it also short circuit the main detection loop: For PV, only
the base group of leaves can possibly hold hypervisor information.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>