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