xive: Make some device types not user creatable

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

$ ppc64-softmmu/qemu-system-ppc64 -device help | grep xive
name "xive-end-source", desc "XIVE END Source"
name "xive-source", desc "XIVE Interrupt Source"
name "xive-tctx", desc "XIVE Interrupt Thread Context"

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 xive-source: required link 'xive' not found:
 Property '.xive' not found
qemu-system-ppc64: -device xive-end-source: required link 'xive' not found:
 Property '.xive' not found
qemu-system-ppc64: -device xive-tctx: required link 'cpu' not found:
 Property '.cpu' not found

Hide them by setting dc->user_creatable to false in their respective
class init functions.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <157017473006.331610.2983143972519884544.stgit@bahia.lan>
Message-Id: <157045578401.865784.6058183726552779559.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
[dwg: Folded comment update into base patch]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>