Add a bus_get_domain() wrapper around BUS_GET_DOMAIN(). Use this to add
a new per-device '%domain' sysctl node that returns the NUMA domain a
device is associated with if it is associated with one.

Note that this API is still a WIP and might change before 11.0 actually
ships.

Differential Revision: https://reviews.freebsd.org/D930
Reviewed by: kib, adrian