vm: document disk stats collection on recovery

The disk handling flow in the recovery path may be
surprising, due to the interaction of moving parts.

Until we properly clean up and refactor this flow,
add a comment to document the behaviour.

Change-Id: Ibd7d32373f5a0a7dd4ef88c5a0ad5a9f3d52938c
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32406
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>