virttest.utils_libguestfs: Add mount option and methods for Guestfish.

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

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs: Makes libguestfs more verbose and repairs __slots__.

Signed-off-by: Jiří Župka <jzupka@redhat.com>