virt: enhancement of class utils_libguestfs.LibguestfsBase.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_libguestfs: Add guestfish about classes.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Add a sample test about guestfish.

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