CLOUDSTACK-6965: fixed the NullPointerException introduced by fix for cloudstack
6935 in AbstractStoragePoolAllocator#filter method for Zone Wide storage

(cherry picked from commit ef45f06f88c955dcf47f1fb69a3a0e87eb817e1d)
CLOUDSTACK-6968: Allowing cluster scope volumes to attach to any VM. If migration is
needed then first they will be migrated to appropriate cluster before attaching.

(cherry picked from commit e7ba46b5f7da21c4fc13dc3284aa802d177045f2)