userportal: memory leak in extended resources

- Fixed memory leak in VmTable, it was holding onto
references to the Vm entities when clearing the Tree.
- Fixed memory leak in AbstractUiCommonModelEditorDriver adding
a visitor on each refresh.

Change-Id: Ie6a6fff003ae1315291de2add063047c14803e75
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=913142
Signed-off-by: Alexander Wels <awels@redhat.com>