hw/ppc/prep: Fix implicit creation of "-drive if=scsi" devices

hw/ppc/prep: Fix implicit creation of "-drive if=scsi" devices

The global hack for creating SCSI devices has recently been removed,
but this apparently broke SCSI devices on some boards that were not
ready for this change yet. For the 40p machine you now get:

$ ppc64-softmmu/qemu-system-ppc64 -M 40p -cdrom x.iso
qemu-system-ppc64: -cdrom x.iso: machine type does not support if=scsi,bus=0,unit=2

Fix it by providing a lsi53c810_create() function that takes care
of calling scsi_bus_legacy_handle_cmdline() after creating the
corresponding SCSI controller.

Fixes: 1454509726719e0933c800fad00d6999752688ea
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>