virtio: verify that legacy support is not accidentally on

virtio: verify that legacy support is not accidentally on

If a virtio device does not have legacy support, make sure that
it is actually off, and bail out if not.

For virtio-pci, this means that any device without legacy support
that has been specified to modern-only (or that has been forced
to it) will work.

For virtio-ccw, this duplicates the check that is currently done
prior to realization for any device that explicitly specified no
support for legacy.

This catches devices that have not been fenced properly.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200707105446.677966-3-cohuck@redhat.com>
Cc: qemu-stable@nongnu.org
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>