virnetdaemon: Extract autoShutdownTimer operations from virNetDaemonRun

virnetdaemon: Extract autoShutdownTimer operations from virNetDaemonRun

Introduce 'virNetDaemonShutdownTimerRegister' and
'virNetDaemonShutdownTimerUpdate' to aggregate the code to deal with the
auto-shutdown timer.

The code is also placed so that it can be called from
'virNetDaemonAutoShutdown' which involved the move of
'virNetDaemonAutoShutdownTimer'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>