Register qemu-xtensa and qemu-xtensaeb for transparent linux userspace
emulation.

Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>