Add VIRTIO_PCI_FLAG_DISABLE_LEGACY and VIRTIO_PCI_FLAG_DISABLE_MODERN
for VirtIOPCIProxy->flags.  Also add properties for them.  They can be
used to disable modern (virtio 1.0) or legacy (virtio 0.9) modes.

By default only legacy is advertized, modern will be turned on by
default once all remaining spec compilance issues are addressed.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>