watchdog: check watchdog device need machine for aarch64

The check cmd is "qemu-kvm -M virt -watchdog '?'" for aarch64.
Signed-off-by: NChen Qun <kuhn.chenqun@huawei.com>

qemu/tests/cfg/watchdog.cfg

@@ -3,6 +3,8 @@
     enable_watchdog = yes
     start_vm = no
     type = watchdog
+    aarch64:
+        watchdog_type_check = " -M virt -watchdog '?'"
     variants:
         -i6300esb:
             no RHEL.4