virttest.utils_libguestfs: Add methods to Guestfish class for volume.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_test: Expand libguestfs module for guestfish volume test.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Add test module for guestfish volume operations.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Replace run_xxx() of test module with run().

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1152 from yumingfei/guestfs_volume_oper

Libguestfs: Add test for guestfish volume operations.