apic: Move hypervisor detection of x2apic to hypervisor.h

Then we can reuse it for Xen later.

Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Avi Kivity <avi@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>