Make getAllVolumes() not fail on broken images.

getAllVolumesImages differentiates between legal and remnants.
Added getAllRemnants for garbage detection.

Related to: BZ#905938, BZ#910013, BZ#875708

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=917363

Change-Id: Ib8514236a5d4793f66709e9daf546fb46047414f
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12546
Tested-by: Daniel Paikov <paikov@gmail.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>