webadmin: Disable pooled VM disks' illegal updates

Disable editing the RO and WipeAfterDelete properties of disks when editing
a disk of a VM that's attached to a VM pool.

Change-Id: I9d12cd00ff1e56bfb44e60fe7e50d66bef630820
Bug-Url: https://bugzilla.redhat.com/1057721
Signed-off-by: Vered Volansky <vvolansk@redhat.com>