x86/apic: Factor out default vector_allocation_domain() operation

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/20120607131449.GC4759@dhcp-26-207.brq.redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>