vhost: use binary search instead of linear in find_region()

For default region layouts performance stays the same
as linear search i.e. it takes around 210ns average for
translate_desc() that inlines find_region().

But it scales better with larger amount of regions,
235ns BS vs 300ns LS with 55 memory regions
and it will be about the same values when allowed number
of slots is increased to 509 like it has been done in kvm.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>