xics: Make some device types not user creatable

Some device types of the XICS model are exposed to the QEMU command
line:

$ ppc64-softmmu/qemu-system-ppc64 -device help | grep ic[sp]
name "icp"
name "ics"
name "ics-spapr"
name "pnv-icp", desc "PowerNV ICP"

These are internal devices that shouldn't be instantiable by the
user. By the way, they can't be because their respective realize
functions expect link properties that can't be set from the command
line:

qemu-system-ppc64: -device icp: required link 'xics' not found:
 Property '.xics' not found
qemu-system-ppc64: -device ics: required link 'xics' not found:
 Property '.xics' not found
qemu-system-ppc64: -device ics-spapr: required link 'xics' not found:
 Property '.xics' not found
qemu-system-ppc64: -device pnv-icp: required link 'xics' not found:
 Property '.xics' not found

Hide them by setting dc->user_creatable to false in the base class
"icp" and "ics" init functions.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <157017826724.337875.14822177178282524024.stgit@bahia.lan>
Message-Id: <157045578962.865784.8551555523533955113.stgit@bahia.lan>
[dwg: Folded reason comment into base patch]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>