vt: register the memory regions used by the vt drivers

Current VT drivers don't register the memory regions they use with the
nexus. This patch makes vt_vga and vt_efifb register the memory regions they
use.

This is needed (at least) for Xen support, since the FreeBSD kernel will try
to use the holes in the memory map to map memory from other domains and
setup it's grant table.

Sponsored by: Citrix Systems R&D
Reported by: sbruno
Tested by: emaste
Reviewed by: ray
PR: 195537
Differential Revision: https://reviews.freebsd.org/D1291