x86/apic: Add apic->eoi_write() callback

Add eoi_write callback so that kvm can override
eoi accesses without touching the rest of the apic.
As a side-effect, this will enable a micro-optimization
for apics using msr.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: gleb@redhat.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/0df425d746c49ac2ecc405174df87752869629d2.1337184153.git.mst@redhat.com
[ tidied it up a bit ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/kernel/apic/apic_noop.c b/arch/x86/kernel/apic/apic_noop.c
index 634ae6c..a6e4c6e 100644
--- a/arch/x86/kernel/apic/apic_noop.c
+++ b/arch/x86/kernel/apic/apic_noop.c
@@ -181,6 +181,7 @@
 
 	.read				= noop_apic_read,
 	.write				= noop_apic_write,
+	.eoi_write			= noop_apic_write,
 	.icr_read			= noop_apic_icr_read,
 	.icr_write			= noop_apic_icr_write,
 	.wait_icr_idle			= noop_apic_wait_icr_idle,