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.
* src/util/buf.c: if virBufferEscapeString was called on a buffer that
had 0 bytes of space, a size of -1 will be passed to snprintf, resulting
in a segmentation fault, this preallocate some space.