xen: xenbus: quirk uses x86 specific cpuid

This breaks on ARM. This quirk is not necessary on ARM because no
hypervisors of that vintage exist for that architecture (port is too
new).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[v1: Moved the ifdef inside the function per Jan Beulich suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>