webadmin: Fix NPE in New VM dialog

Regression caused by
416cd92a94bbc95c8e5bca17f3707d2016208668
which expected that the selected item is
always set in VmListModel.onSave(). The problem was
that it is true only for edit VM - not for new.

Change-Id: I1e653f428f8a058532030483e7bb115961d706c3
Signed-off-by: Martin Betak <mbetak@redhat.com>