virtio-mmio: add support for scsi devices.

virtio-mmio: add support for scsi devices.

Add new fields to struct virtio_lun_s for mmio support,
add mmio parameter to virtio_scsi_init_lun(), so both
pci and mmio devices can be handled.

Add and use bootprio_find_scsi_mmio_device() to figure
boot priority of devices connected to a virtio-mmio
scsi controller.

Finally add init_virtio_scsi_mmio() to initialize one
virtio-mmio scsi controller.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>