webadmin: edit unattached SD - prevent infinite progress

StorageModel -> postUpdateHost:
When editing a storage domain, hosts are filtered
according to the SPM. If no SPM is found, 'hosts'
list can simply be set to an empty list (instead
of a list with a single null object; which
causes an NPE later on).

Change-Id: Iad4b040ab1de8db42f43defbad0396ef4013572e
Bug-Url: https://bugzilla.redhat.com/1017182
Signed-off-by: Daniel Erez <derez@redhat.com>