spice: init dcl before registering qxl interface

Without this spice might callback into qemu before ssd->dcl.con is
initialized, resulting in a segfault due to NULL pointer dereference.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
