PPC: Get rid of segfaults in DBDMA emulation

While trying to find the right channel number for the DBDMA emulation I
stumbled across segmentation faults that were purely triggered by the guest.

The guest should never have the possiblity to segfault us, so let's check
all indirect function calls on a channel, so the code even works for channels
that have not been reserved.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
