hw/intc/xics: Add missing call to register vmstate_icp_server

hw/intc/xics: Add missing call to register vmstate_icp_server

An obsolete wrapper function with a workaround was removed entirely,
without restoring the call it wrapped.

Without this, the guest is stuck after savevm/loadvm.

Fixes: 24ee9229fe31 ("ppc/spapr: remove deprecated machine pseries-2.9")
Signed-off-by: Fabian Vogt <fvogt@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/6187781.lOV4Wx5bFT@fvogt-thinkpad
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Link: https://lore.kernel.org/r/20250819223905.2247-2-farosas@suse.de
Message-ID: <20250819223905.2247-2-farosas@suse.de>
(cherry picked from commit f5738aedc21790bd07dbead6b6272a605d5c1138)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>