virttest.libguestfs: Unified virt-command functions.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_libguestfs: Add virt command function.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libguestfs: Add functions for formatting disk.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libguestfs: Add inspective functions for VT class.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1440 from yumingfei/virt_part_oper

virttest.libguestfs: Add functions for libguestfs test module.