core: Use audit log only when failed to use retrieve command.

Using a warning audit log only when the retrieve of OVF_STORE disks has
been failed.

When there are no OVF_STORE disks available we should only print a
warnning log which indicates that there were no OVF_STORE in the storage
domain.

Change-Id: I1eb5a687429c081f42bdc63bf3b8dd17eb295236
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>