libvirt.tests: Fix some problems of storage_discard

    
    *Always closing session after logining to vm.
    *Change timeout to allow fstrim with large disks.
    *Use a new vm for easy cleanup.
    
    * For new version, the output will include human-readable format(Mib)
      Force to use bytes to parse the fstrimmed size.
    * Increase the timeout of fstrim because for large device(>1G),
      it may take more time.
    * Output the result of vgthin for debug.

    *libvirt.tests: Fix attributes' error of channel.