hotplug_mem.cfg: edit threshold adapt to arm64-pci machine type.

Edit threshold adapt to arm64-pci machine type, to make VM memory size as
expected.

Signed-off-by: Wang Ziming <wangziming.huawei.com>

qemu/tests/cfg/hotplug_mem.cfg

@@ -19,8 +19,8 @@
     # Notes:
     #    The threshold on ppc was confirmed with developer
     #    since crashkernel size is much bigger than x86
-    ppc64,ppc64le:
-        threshold = 0.15
+    ppc64,ppc64le,aarch64:
+        threshold = 0.20
     sub_test_wait_time = 0
     variants:
         - @default: