cpu_device_hotplug: Add 'aarch64' to supported platform in cpu_device_hotplug.cfg.

Arm machine can run these testcases, when testing large format VM, in case
of out of memory happened, set VM memory as 16384 MiB。Besides, aarch64 vm
vcpu topology only support 'threads=1'.
Signed-off-by: NJie Tang <tangjie18@huawei.com>

qemu/tests/cfg/cpu_device_hotplug_maximum.cfg

@@ -3,8 +3,9 @@
     type = cpu_device_hotplug_maximum
     no ovmf
     no RHEL.6
-    only x86_64 ppc64 ppc64le
+    only x86_64 ppc64 ppc64le aarch64
     required_qemu = [2.6.0, )
+    mem = 16384
     start_vm = no
     qemu_sandbox = on
     allow_pcpu_overcommit = yes
@@ -25,6 +26,7 @@
             vcpu_cores = 0
             vcpu_threads = 1
         - max_thread:
+            only x86_64 ppc64 ppc64le
             vcpu_sockets = 1
             vcpu_cores = 1
             vcpu_threads = 0

qemu/tests/cfg/cpu_device_hotpluggable.cfg

 # Notes:
 #    For ppc64/ppc64le, please manually specify cpu_model in your environment
 - cpu_device_hotpluggable: install setup image_copy unattended_install.cdrom
-    only ppc64 ppc64le x86_64
+    only ppc64 ppc64le x86_64 aarch64
     Windows:
         no WinXP, WinVista, Win7, Win8, Win10, Win2000, Win2003
     # ovmf does not support hotpluggable vCPU yet, this line will be removed
@@ -70,6 +70,7 @@
         - single_vcpu:
             vcpu_devices = vcpu1
         - multi_vcpu:
+            vcpu_maxcpus = 16
             vcpu_devices = vcpu1 vcpu2 vcpu3 vcpu4
     variants:
         - hotplug: