webadmin: disable quota selection in edit vm pool

quota selection should not be editable when editing a vm pool.

fixed the selection box editability.

Change-Id: Id5778ee37b1086618608edc5b470a1dbe6b5b3d8
Bug-Url: https://bugzilla.redhat.com/966003
Signed-off-by: Ofri Masad <omasad@redhat.com>