clientIF: Teardown volume path only for VDSM images

Trying to tear down a cdrom or external lun throws an exception which
creates a lot of log printings.

Change-Id: I041a306636c75a7aa37d4d7c0811366d80fe609c
Bug-Url: https://bugzilla.redhat.com/980054
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21973
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>