core: Audit log severity for live extend

The last step of extending a disk is notifying Qemu, so it can take
advantage of the new size. If this notification fails, an audit log is
produced.

This patch changes this audit log's severity to WARNING, so it is more
visible to the user.

Change-Id: Ic7ccfe3975bc410fc3e144141c066ad6c0378c12
Bug-Url: https://bugzilla.redhat.com/1085359
Signed-off-by: Allon Mureinik <amureini@redhat.com>