webadmin: make storage type field non editable

In edit dialog of storage domain
make the "Domain Function / Storage Type" field not editable.
It remains editable when adding a new storage domain.

Change-Id: I7c6c3db97ca8a2d07ec7ec50b0765828b902e5a8
Signed-off-by: Alissa Bonas <abonas@redhat.com>