Impact: new API

Add a helper function that takes two cpumask's, and's them and then
returns the apicid of the result.  This removes a need in io_apic.c
that uses a temporary cpumask to hold (mask & cfg->domain).
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>