core: WARNING when failed to create OVF Store

Changed audit log severity to WARNING to indicate that there's a problem
here.

Change-Id: I78171fd1890435c9f98efec88496e81e5f84b9ce
Bug-Url: https://bugzilla.redhat.com/1134371
Signed-off-by: Allon Mureinik <amureini@redhat.com>