[Libguestfs] [PATCH v2 0/4] common/utils: Move libxml2 writer macros to a common header file.

v1 was here: https://www.redhat.com/archives/libguestfs/2018-October/msg00047.html However it was broken in a few ways. First of all the documentation was broken because "/**" enhanced comments were not permitted on macros. This is fixed in the new 1/4 patch. Secondly we didn't use single_element() everywhere possible, which is fixed in the new 4/4 patch. Lastly I've enhanced the documentation so it's clear what precise XML is generated. Rich.