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>