virtio: introduce virtio_legacy_is_cross_endian()

This helper will be used by vhost and tap to detect cross-endianness in
the legacy virtio case.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>