virCgroupKillRecursive sneakily initializes 'ret' to 0 rather than the
usual -1. 401030499bf moved an error condition but didn't actually
modify 'ret' return the proper error code.
Fixes: 401030499bf Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Pavel Hrdina <phrdina@redhat.com>