Avoid CPU endian memory accesses in devices

Don't compile virtio.c in hwlib, it depends on memory accesses
performed in CPU endianness.

Make loads and stores in CPU endianness unavailable to devices
and poison them to avoid further bugs.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>