Revert "hw: report invalid disable-legacy|modern usage for virtio-1-only devs"

Revert "hw: report invalid disable-legacy|modern usage for virtio-1-only devs"

This reverts commit f2784eed306449c3d04a71a05ed6463b8289aedf
since that accidentally removes the PCIe capabilities from virtio
devices because virtio_pci_dc_realize is called before the new 'mode'
flag is set.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190729162903.4489-3-dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>