webadmin: Use GetEntity instead of storageDomain

The storageDomain selected in the GUI when detaching a local Storage
Domain intialized with storage pool id and therefore will fail the
operation of detach local Storage Domain.
Instead using storageDomain we should use getEntity

Change-Id: I594cee0e9130869b0c3ff55b6ecd004f8a42f61e
Bug-Url: https://bugzilla.redhat.com/1148417
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>