virttest.virsh: Add pool_build function.

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

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest: Add unittest for libvirt_storage module.

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