core: Increment DB Generation when image is copied

Increment the DB genereation when a template image is being copied, so
the desctination Storage Domain will be updated with the Template OVF.

Change-Id: Icdd5e02387fe729195bd4b3c502636671a914e5d
Bug-Url: https://bugzilla.redhat.com/1138136
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>