s390/stattrib: Add Error** argument to set_migrationmode() handler

s390/stattrib: Add Error** argument to set_migrationmode() handler

This will prepare ground for future changes adding an Error** argument
to the save_setup() handler. We need to make sure that on failure,
set_migrationmode() always sets a new error. See the Rules section in
qapi/error.h.

Cc: Halil Pasic <pasic@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240320064911.545001-2-clg@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>