engine: add comment field to VM,Template,Pool [6]

    As part of the Free Text RFE

    Added to the entities flow in the system
    DB <-> DAO <-> Entity <-> Model <-> UI
    DB <-> DAO <-> Entity <-> REST

    Sadly there is no way to split the change between the 3 entities
    in a way that will not introduce breaking changes to master, so I pushed them together.

Change-Id: I1f8c0af979e9ec4724284d92f79a0b688e961425
Signed-off-by: Noam Slomianko <nslomian@redhat.com>