Replace free(virBufferContentAndReset()) with virBufferFreeAndReset().
Update documentation and replace all remaining calls to free() with
calls to VIR_FREE(). Also add missing calls to virBufferFreeAndReset()
and virReportOOMError() in OOM error cases.
This patch is the result of running the following command in the docs
directory: sed -i 's/\t/ /g; s/\s*$//' *.html.in
* docs/*.html.in:convert tabs into 8 spaces and remove trailing whitespace