x86/apic: Provide apic_update_callback()

There are already two variants of update mechanism for particular callbacks
and virtualization just writes into the data structure.

Provide an interface and use a shadow data structure to preserve callbacks
so they can be reapplied when the APIC driver is replaced.

The extra data structure is intentional as any new callback needs to be
also updated in the core code. This also prepares for static calls.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)