libvirt-tests: Backup inactive domain XML

* Added option to toggle saving active or inactive domain XML for libvirt_vm.backup_xml

* Added '--inactive' option when dumping xml for backup when calling vmXML.new_from_dumpxml, 
  virsh.dumpxml or libvirt_vm.backup_xml. Without this option, the new domain restored will change 
  its inactive xml like following diff permanently.

```diff
@@ -41,7 +41,9 @@
       <target type='serial' port='0'/>
     </console>
     <input type='mouse' bus='ps2'/>
-    <graphics type='vnc' port='-1' autoport='yes'/>
+    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1'>
+      <listen type='address' address='127.0.0.1'/>
+    </graphics>
     <video>
       <model type='cirrus' vram='9216' heads='1'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
@@ -50,4 +52,5 @@
       <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
     </memballoon>
   </devices>
+  <seclabel type='dynamic' model='selinux' relabel='yes'/>
 </domain>
```