v2v.testcases: Enhancements of convert_remote_vm.

* Target_dev should be same as lvm pool's name.
* Replace test.tmpdir to data_dir to avoid too long
  path error.
* Add logic volume checking for creating lvm pool.
* Print more debug info for test.
Signed-off-by: NYu Mingfei <yumingfei@cn.fujitsu.com>