virttest.virsh: Use pool_name instead of extra to pass pool.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.libvirt_storage: Add PoolVolume class

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1425 from autotest/1405

PR 1405 remake - PoolVolume Class