Refactor volume#to_xml to please RuboCop

Fixes: 476c630 ("Always use XML builders to construct XML")

Author: ekohl

lib/fog/libvirt/models/compute/volume.rb

@@ -92,26 +92,14 @@ def to_xml
 
               xml.target do
                 xml.format(:type => format_type)
-
-                xml.permissions do
-                  xml.owner(owner) if owner
-                  xml.group(group) if group
-                  xml.mode('0744')
-                  xml.label('virt_image_t')
-                end
+                xml_permissions(xml)
               end
 
               if backing_volume
                 xml.backingStore do
                   xml.path(backing_volume.path)
                   xml.format(:type => backing_volume.format_type)
-
-                  xml.permissions do
-                    xml.owner(owner) if owner
-                    xml.group(group) if group
-                    xml.mode('0744')
-                    xml.label('virt_image_t')
-                  end
+                  xml_permissions(xml)
                 end
               end
             end
@@ -122,6 +110,15 @@ def to_xml
 
         private
 
+        def xml_permissions(xml)
+          xml.permissions do
+            xml.owner(owner) if owner
+            xml.group(group) if group
+            xml.mode('0744')
+            xml.label('virt_image_t')
+          end
+        end
+
         def image_suffix
           return "img" if format_type == "raw"
           format_type