tests: rename target_big_endian() as qvirtio_is_big_endian()

tests: rename target_big_endian() as qvirtio_is_big_endian()

Move the definition to libqos/virtio.h as it must be used
only with virtio functions.

Add a QVirtioDevice parameter as it will be needed to
know if the virtio device is using virtio 1.0 specification
and thus is always little-endian (to do)

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>