xen/x86: Implement x86_apic_ops

Or rather just implement one different function as opposed
to the native one : the read function.

We synthesize the values.

Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
[v1: Rebased on top of tip/x86/urgent]
[v2: Return 0xfd instead of 0xff in the default case]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>