webadmin: fix storage QoS UI validation

The storage values weren't validated.
Also added a check to allow setting total or read/write.

Change-Id: Ia4f69ce1b7751e066c6a1e15f482805bbc7df16a
Bug-Url: https://bugzilla.redhat.com/1168558
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>