core: fix VmTemplate copy ctor

remove code duplication by calling VmBase(VmBase vmBase) ctor that
already copies all the field from that class, and leave in VmTemplate
only the template fields.

Change-Id: Ie585d00ee6c4232b09e1a2396ad335348b8122be
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>