userportal: UnitVmModel - subscribe GetHostsByClusterId

Adding VdcQueryType.GetHostsByClusterId to subscribed
query types in Frontend (for preventing progress 'blink'
on new/edit VM dialog).

Change-Id: If500f79ee46638dc2a94a73cf673d8bdbd1d8880
Signed-off-by: Daniel Erez <derez@redhat.com>