core: add 'Comment' field to OVF

'Comment' is missing from the OVF, therfore, missing from snapshots,
import-export and also not working when editing running vm.

this patch add the fields to the OVF and solving all above.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1128446
Change-Id: I79fdcfd10d2bd510b57314f330f349b38e6f5ee2
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>