arm: use migrate_once() in migration tests

Some tests already shipped with their own do_migrate() function, remove
it and instead use the new migrate_once() function. The control flow in
the gic tests can be simplified due to migrate_once().

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Message-Id: <20221212111731.292942-5-nrb@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>