getloadavg is supported on Linux, BSDs, Solaris.

Following man page:
RETURN VALUE
       If the load average was unobtainable, -1 is returned; otherwise,
       the number of samples actually retrieved is returned.

accordingly, make stub for systems which don't support this function return -1
for consistency.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>