Fix unit tests for libvirt 2.5+

In libvirt v2.5+, domain.destroy() raises an error in unit tests:
"Requested operation is not valid: domain is not running"

- check if the domain is active in the unit tests.

Change-Id: I92420699738ea3e5ece4e9137ac797fbcbc015ea
Related-bug:#1641568

devops/tests/driver/libvirt/base.py

@@ -123,7 +123,8 @@ class LibvirtTestCase(TestCase):
         for domain in conn.listAllDomains():
             for snapshot in domain.listAllSnapshots(0):
                 snapshot.delete()
-            domain.destroy()
+            if domain.isActive():
+                domain.destroy()
             domain.undefine()
         for network in conn.listAllNetworks():
             network.destroy()