webadmin: disks -  prevent redundant search manipulation

Ensure that the selected main tab (in commonModel) is DiskListModel.

Change-Id: I0505bbea25e16f7e663689f4baa66df103e8b533
Bug-Url: https://bugzilla.redhat.com/1073574
Signed-off-by: Daniel Erez <derez@redhat.com>