Partially revert "vt: register the memory regions used by the vt drivers"

Revert the EFI part of r276064 until I can test it properly on a real EFI
system. This was causing problems to people booting using UEFI and vt.

Reported by:	O. Hartmann <ohartman@zedat.fu-berlin.de>
vt_vga: fix an off-by-one error

This correctly sets the end of the memory region registered by vt_vga.

Reported by:	marius
Allow systems having a page size greater than 4K to use fewer
scatter-gather XHCI TRB entries for its payload data. The XHCI
controller can handle at least 65536 bytes per scatter-gather list
entry.

MFC after:	1 week
Suggested by:	Kohji Okuno <okuno.kohji@jp.panasonic.com>