Improve virConf parse to handle LXC config format

Improve virConf parse to handle LXC config format

virConf now honours a VIR_CONF_FLAG_LXC_FORMAT flag to handle LXC
configuration files. The differences are that property names can
contain '.' character and values are all strings without any bounding
quotes.

Provide a new virConfWalk function calling a handler on all non-comment
values. This function will be used by the LXC conversion code to loop
over LXC configuration lines.