qga: implement a 'guest-get-load' command

qga: implement a 'guest-get-load' command

Provide a way to report the process load average, via a new
'guest-get-load' command.

This is only implemented for POSIX platforms providing 'getloadavg'.

Example illustrated with qmp-shell:

(QEMU) guest-get-load
{
    "return": {
        "load15m": 1.546875,
        "load1m": 1.669921875,
        "load5m": 1.9306640625
    }
}

Windows has no native equivalent API, but it would be possible to
simulate it as illustrated here (BSD-3-Clause):

  https://github.com/giampaolo/psutil/pull/1485

This is left as an exercise for future contributors.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-ID: <20241202121927.864335-1-berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>