engine: Persist comment in AddVmTemplateCommand

AddVmTemplateCommand did not map comment field from given VM to newly
created template.

Also added corresponding mapping to frontend CommonUnitToVmBaseBuilder.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1013670
Change-Id: I5ef17d46b067036063865fbb0e64d266493132cb
Signed-off-by: Martin Betak <mbetak@redhat.com>