core: Add a warnning when OVF_STORE disks does not exists.

Add a warnning on attach operation to notify the user that there are no
backup componenets if the Storage Domain was handled before in a Data
Center version lower then 3.5

Change-Id: I79de5110898852a5bf3d46227aa7eef36a74a74a
Bug-Url: https://bugzilla.redhat.com/1168132
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>