migration/colo/xen: Use generic helpers in qemu_save_device_state()

migration/colo/xen: Use generic helpers in qemu_save_device_state()

Use qemu_savevm_state_non_iterable*() helpers for saving device states,
rather than walking the vmstate handlers on its own.

Non-iterables can be either early_setup devices, or otherwise.

Note that QEMU only has one early_setup device currently, which is
virtio-mem, and I highly doubt if it is used in either COLO or Xen users..
However this step is still better needed to provide full coverage of all
non-iterable vmstates.

When at it, allow it to report errors.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Paul Durrant <paul@xen.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Lukas Straub <lukasstraub2@web.de>
Link: https://lore.kernel.org/qemu-devel/20260127185254.3954634-25-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>