virttest.libvirt_xml: Add get_first_mac_by_name method for VMXML class.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_libguestfs: Enhancements for module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest: Add utils_test module for libguestfs.

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

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