webadmin: Allow editing unplugged disk in UI

"Edit" button under VM => Disks is disabled after disk was unplugged
from VM. It should be enabled as it was before 3.3.

Change-Id: Ia128fe1dbe74be608d27737146d575c52e131c56
Bug-Url: https://bugzilla.redhat.com/988061
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>