xen/x86: Implement x86_apic_ops

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.

[upstream git commit 31b3c9d723407b395564d1fff3624cc0083ae520]
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>
Conflicts:

	arch/x86/xen/Makefile
	arch/x86/xen/enlighten.c
	arch/x86/xen/xen-ops.h
[In the makefile, the vga.o didn't exist in 3.0 - it was added
in 3.1]