ui: only allow rbd pools to be added as rbd storage

ui: only allow rbd pools to be added as rbd storage

previously the ui would allow adding all pools (even the default
ceph-mon pools) as storage. this could lead to issues when users did
use these pools as storage (e.g.: vms missing their disks after a
migration). hence, restrict the pool selector to rbd pools.

fails gracefully by reverting to the previous behavior if a pool has
no application assigned to it.

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>
Tested-By:  Aaron Lauterer <a.lauterer@proxmox.com>