cpu_device_hotplug_maximum: The value of machine_type getted from machines.cfg is incorrect on arm.

The value of machine_type getted from machines.cfg is incorrect on arm, it should be cut before used.
Signed-off-by: NJie Tang <tangjie18@huawei.com>

qemu/tests/cpu_device_hotplug_maximum.py

 import re
 import logging
+import platform
 from os import uname
 
 from avocado.utils import cpu
@@ -28,7 +29,10 @@ def run(test, params, env):
     :param env:    Dictionary with test environment.
     """
     os_type = params["os_type"]
-    machine_type = params["machine_type"]
+    if platform.machine() == 'aarch64':
+        machine_type = params["machine_type"].split(':', 1)[1]
+    else:
+        machine_type = params["machine_type"]
     reboot_timeout = params.get_numeric("reboot_timeout")
     mismatch_text = "Actual number of guest CPUs is not equal to the expected"
     not_equal_text = "CPU quantity mismatched! Guest got %s but expected is %s"