xics: Link ICP_PROP_XICS property to ICPState::xics pointer

xics: Link ICP_PROP_XICS property to ICPState::xics pointer

The ICP object has both a pointer and an ICP_PROP_XICS property pointing
to the XICS fabric. Confusing bugs could arise if these ever go out of
sync.

Change the property definition so that it explicitly sets the pointer.
The property isn't optional : not being able to set the link is a bug
and QEMU should rather abort than exit in this case.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <157403284152.409804.17114564311521923733.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>