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
  virsh.dumpxml or libvirt_vm.backup_xml. Without this option, the new
  domain restored will change its inactive xml like following diff permanently.

* using vmXML.new_from_inactive_xml instead of vmXML.new_from_xml to
  backup domain.

```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>
```

Signed-off-by: Hao Liu <hliu@redhat.com>