vhost: simplify vhost_needs_vring_endian()

After the call to virtio_vdev_has_feature(), we only care for legacy
devices, so we don't need the extra check in virtio_is_big_endian().

Also the device_endian field is always set (VIRTIO_DEVICE_ENDIAN_UNKNOWN
may only happen on a virtio_load() path that cannot lead here), so we
don't need the assert() either.

This open codes the device_endian checking in vhost_needs_vring_endian().
It also adds a comment to explain the logic, as recent reviews showed the
cross-endian tweaks aren't that obvious.

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>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
