fix(instances): enabling vCPU hot plug keeps the current vCPU count

Enabling hot plug on a stopped VM wrote every vCPU up to the maximum as
enabled, so a VM with 2 of 4 vCPUs booted with 4. The current vCPUs stay
enabled now; libvirt adds the rest as disabled hotpluggable vCPUs.

vrtManager/test_device_edits.py

@@ -1,7 +1,8 @@
 """Device edits of a VM: each definition is changed as it describes the
 device, every NIC type can be a boot device, posted values stay values, and
 definition changes go through _defineXML. An active VM's memory is resized
-by balloon, a stopped one's in its definition."""
+by balloon, a stopped one's in its definition. Enabling vCPU hot plug keeps
+the vCPUs enabled at boot."""
 
 import unittest
 from unittest.mock import MagicMock
@@ -65,6 +66,15 @@ class DetachDiskTestCase(unittest.TestCase):
         p.instance.detachDeviceFlags.assert_not_called()
 
 
+class VcpuHotplugTestCase(unittest.TestCase):
+    def test_enabling_hotplug_keeps_the_current_vcpus_enabled(self):
+        p = proxy(active=False)
+        p.instance.info.return_value = [5]
+        p._XMLDesc = lambda flags: "<domain><vcpu current='2'>4</vcpu></domain>"
+        p.set_vcpu_hotplug(True)
+        self.assertEqual(defined(p).xpath("vcpus/vcpu/@enabled"), ["yes", "yes"])
+
+
 class DeviceEditsTestCase(unittest.TestCase):
     def test_a_block_volume_is_attached_by_its_device_path(self):
         p = proxy(active=False)