Rename vt(4) vga module to dismiss interference with syscons(4) vga module.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Remove stale link to deleted vt(4) xboxfb driver.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
syscons(4) and vt(4) can be built together now.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation