hw/display/vga-mmio: QOM'ify vga_mmio_init() as TYPE_VGA_MMIO

Introduce TYPE_VGA_MMIO, a sysbus device.

While there is no change in the vga_mmio_init()
interface, this is a migration compatibility break
of the MIPS Acer Pica 61 Jazz machine (pica61).

Suggested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211206224528.563588-4-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>