webadmin: new vm - fix host selection on cluster change

When changing dc/cluster in VM dialog, the selected
host was empty as it dependeds on the ' oldDefaultHost'
Fixed by ensuring that an item from the hosts list
is selected.

Change-Id: Icd80711a9c1557743946445196c3c0015d94fc26
Bug-Url: https://bugzilla.redhat.com/1064393
Signed-off-by: Daniel Erez <derez@redhat.com>