webadmin: NPE fix in CommonModel

Fixed an NPE that prevented the VMs from loading when
Gluster was configured in engine and the VMs item
was selected in the left-pane tree.

Change-Id: I7768bf29a3068a05af624dce1e95f9f2f14f792a
Bug-Url: https://bugzilla.redhat.com/987907
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>