TYPE_ICS_SIMPLE is the only subtype of TYPE_ICS_BASE that's ever
instantiated.  The existence of different classes is mostly a hang
over from when we (misguidedly) had separate subtypes for the KVM and
non-KVM version of the device.

There could be some call for an abstract base type for ICS variants
that use a different representation of their state (PowerNV PHB3 might
want this).  The current split isn't really in the right place for
that though.  If we need this in future, we can re-implement it more
in line with what we actually need.

So, collapse the two classes together into just TYPE_ICS.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>