frontend: Fix display of custom properties on edit VM

In ExistingVmModelBehavior the custom properties sheet was not
deserialized from entity to corresponding widget.

Also on backend when we were dealing with VM that has next runtime
configuration the OvfImport did copy the userDefined and predefined
properties but not the aggregate customProperties.

Change-Id: I523c61595911138b07ee74563239dd80ccb8cc99
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1128433
Signed-off-by: Martin Betak <mbetak@redhat.com>