libvirt.tests: Fix attributes' error of channel.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
storage_discard: Fix fstrim bugs.

* 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.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
storage_discard: Some adjustments for testing.

*Always closing session after logining to vm.
*Change timeout to allow fstrim with large disks.
*Use a new vm for easy cleanup.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #350 from yumingfei/discard

libvirt.tests: Fix some problems of storage_discard