Qubes Manager: Rename "Disk size" to "Disk usage"

What's shown in that column is the disk space that used rather
than the size of the full disk. So, let's be clear about that.

ui/qubemanager.ui

@@ -211,7 +211,7 @@
       </column>
       <column>
        <property name="text">
-        <string>Disk size</string>
+        <string>Disk usage</string>
        </property>
       </column>
       <column>