Tests.vol_create_from: Replace pool operations with libvirt_storage module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Move nfs/iscsi setup&cleanup to utils_test module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Replace volume operations with libvirt_storage.PoolVolume.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Adjust vol_create_from test.

Functions for preparing pool and volume for test is useful
in other test modules. So move it to utils_test.libvirt.

Then in some failed conditions, it will leave many iscsi
devices on host. We should keep it clean.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Fix virsh.vol_create_from test bug.

When creating logical pool, pool_target should be volume
group's path. But it has been fixed to vg_logical in pre_pool,
so target in configuration should be vg_logical too.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Minor changes for vol_create_from test.

1.Change logical pool target to vg_logical to cordinate with source
2.Subtract emulated image size for saving time
3.Debug more clear for created volumes

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #50 from yumingfei/volume_enhancements

Libvirt.tests: Volume enhancements