Add support for detecting lvm vorigin snapshot volumes. (#633038)
    
    We don't show them, don't offer to edit or delete them. All we do is
    account for the space they occupy in the volume group.
    
    This patch also consolidates the calculation of snapshot space usage
    in LVMVolumeGroupDevice into a property, snapshotSpace, and refactors
    LVMVolumeGroup.freeSpace accordingly.