virttest.utils_libguestfs: Enhancements of virt_edit interface.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1609 from yumingfei/virt_edit

virttest.utils_libguestfs: Enhancements of virt_edit interface.
utils_net: add method dellink for Interface class

Implement dellink method in Interface using netlink.

Signed-off-by: Xiangmin Han <xhan@redhat.com>