mkdumprd: do not lookup in by-uuid dirs for raw device's persistent name
    
    raw devices are not mounted and also does not need to contain any
    filesystem. So they may have UUIDs(when formatted) and may not have UUIDs
    when raw. Therefore, do not look for persistent names by-uuid for raw
    devices.
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Suggested-by: Dave Young <dyoung@redhat.com>
    Acked-by: Xunlei Pang <xlpang@redhat.com>
    Acked-by: Baoquan He <xlpang@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>