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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274976 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f