core: add get*Type to template DAO

Add methods in VmTemplateDAO to get specific types Instance and Image,
using these will assure the user that the object return is of the right
type.

Change-Id: Ic9f0eef1e61b51cc941b8dd9403f5733a69f9dfc
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>