webadmin: disks tab - fix type selection after sign out

MainTabDiskView -> use CommonModelManager singleton instance
instead of saving it as a class member (needed for fixing disk
type selection after signing out).

Change-Id: I11b1489db03608fddf1aa24fa6664f6c3ab5abb3
Bug-Url: https://bugzilla.redhat.com/1073574
Signed-off-by: Daniel Erez <derez@redhat.com>