webadmin: removed dead code from VmGeneralModel

VmGeneralModel -> updateStorageDomain method has been
used for displaying in which domain the VM resides on.
Since a VM can contain disks from multiple storage domains
this method is no longer relevant.

Change-Id: Ida307e9bf7d8a89d6466569f16ae0596c648d636
Signed-off-by: Daniel Erez <derez@redhat.com>