qemuProcessLaunch: Setup handling of 'on_reboot' via QMP when starting the process

qemuProcessLaunch: Setup handling of 'on_reboot' via QMP when starting the process

Rather than using '-no-reboot' use the QMP command to update the
lifecycle action of 'on_reboot'.

This will be identical to how we set the behaviour during lifetime and
also avoids problems with use of the 'system-reset' QMP command during
bringup of the VM (used to update the firmware table of disks when disks
were hotplugged as part of startup).

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