xen/pv: fix vendor checks for pmu emulation

The CPU vendor checks for pmu emulation are rather limited today, as
the assumption seems to be that only Intel and AMD are existing and/or
supported vendors.

Fix that by handling Centaur and Zhaoxin CPUs the same way as Intel,
and Hygon the same way as AMD.

While at it fix the return type of is_intel_pmu_msr().

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>