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>

arch/x86/xen/apic.c

@@ -0,0 +1,17 @@
+#include <linux/init.h>
+#include <asm/x86_init.h>
+
+unsigned int xen_io_apic_read(unsigned apic, unsigned reg)
+{
+	if (reg == 0x1)
+		return 0x00170020;
+	else if (reg == 0x0)
+		return apic << 24;
+
+	return 0xfd;
+}
+
+void __init xen_init_apic(void)
+{
+	x86_io_apic_ops.read = xen_io_apic_read;
+}