webadmin: MoveDiskModel - extract SD selection callback

Extract callback of storage domain selection to a method
(for ensuring invocation on initial selection).

Change-Id: I87963e34a6df9e7c2ae6da2950afa490a7bb2afa
Bug-Url: https://bugzilla.redhat.com/1099083
Signed-off-by: Daniel Erez <derez@redhat.com>