vga: set owner for mmio regions

This makes sure the regions are properly cleaned when unplugging -device
seconday-vga.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180626060941.8326-1-kraxel@redhat.com