Version 3 of this patch. Because I now have to pass the 'mgr' pointer
around, the patch is considerably more complicated than before.
Patch 1/2 is required so that I can use virReportSystemError when I
don't need to have any optional arguments, ie. the equivalent of:
printf ("foo\n");
I have tested these two patches against the libguestfs test suite, and
it works for me and removes the memory leak.
Rich.