fence_scsi: replace open3 calls with qx commands
    
    The open3 calls will block when the buffer becomes full, which causes
    fence_scsi to block. This can occur if fence_scsi needs to process
    several devices. All open3 calls should be replaced with qx commands,
    which does not suffer from this problem. (BZ 545193)
    
    Signed-off-by: Ryan O'Hara <rohara@redhat.com>