utils_test.libvirt: Fix PoolVolumeTest.pre_pool bugs.

1.Support to specify source path for nfs creating.
2.Volume Group path will not exist if there is no any volume in.
  So prepare one here to help creating new volumes in test.
3.Nfs server used for creating pool should not be mounted.
  Otherwise, it will be more like dir type pool.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt_storage: output more debug info for create_volume.

There will be no enough information if created volume can
not be found.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>