webadmin: show VMs sub tab only when virt service enabled

  Show Virtual Machine sub tab under Hosts tab only when virt
service is enabled for the respective cluster.

Change-Id: I17a02f648219f8d69a19557e0f22ae1f3c07a50d
Bug-Url: https://bugzilla.redhat.com/960546
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>