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.

* Add quotation marks for write commands' parameters.
* Add some inner functions for GuestfishPersistent.
* Add some virt-commands for utils_libguestfs module.

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

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

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