ide/mmio: QOM'ify MMIO IDE for R2D

It was not qdev'ified before, so turn it into a SysBusDevice.
Keep mmio_ide_init_drives() around to attach the hard drive.

Signed-off-by: Andreas Färberr <afaerber@suse.de>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>