virttest.utils_libguestfs: Add guestmount and virt-list functions

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Add testcases for guestmount command.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Add testcase for virt list functions

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