frontend: old DC used if DC is selected in tree

The problem was in UnitVmModel which used the StoragePool entity from the
system tree and not the one just updated from backend.

Fixed by using the correct one.

Change-Id: I29bd052f7ed6dacd014e0c4348584881ee15ef02
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988018
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>