core: ImportVm - use device info only if supported

vm devices were intoduced only in 3.1
when importing vm, check ovf version to know if to use devices
information from it.

Change-Id: I5f57aad01383b8a3238030b1bae5b6f8a5e1481a
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>