libvirt creates invalid commands if wrong locale is selected. For
example with locale that uses comma as a decimal point, JSON commands
created with decimal numbers are invalid because comma separates the
entries in JSON. Fortunately even when decimal point is affected,
thousands grouping is not, because for grouping to be enabled with
*printf, there has to be an apostrophe flag specified (and supported).

This patch adds specific internal function for converting doubles to
strings with C locale.