Right now the spapr devices cannot be instantiated with -device,
because the IRQs need to be passed to the spapr_*_create functions.
Do this instead in the bus's init wrapper.

This is particularly important with the conversion from scsi-disk
to scsi-{cd,hd} that Markus made.  After his patches, if you
specify a scsi-cd device attached to an if=none drive, the default
VSCSI controller will not be created and, without qdevification,
you will not be able to add yours.

NOTE from agraf: added small compile fix
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NAlexander Graf <agraf@suse.de>