mpath: Update path in CheckPool function
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1230664
    
    Per the devmapper docs, use "/dev/mapper" or "/dev/dm-n" in order to
    determine if a device is under control of DM Multipath.
    
    So add "/dev/mapper" to the virFileExists, leaving the "/dev/mpath"
    as a "legacy" option since it appears for a while it was the preferred
    mechanism, but is no longer maintained