core: add copy constructor to VmTemplate

http://www.ovirt.org/index.php?title=Features/Template_Versions

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1037478
Change-Id: I85fcd67ebba67bcae588420f8ce52268d13bca04
Signed-off-by: Arik Hadas <ahadas@redhat.com>