shell: Don't start plots when they have not been layouted yet
shell: Don't start plots when they have not been layouted yet

This allows the callers to be less careful when calling start().

Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>

vanloswang/cockpit,vbatts/cockpit,vanloswang/cockpit,arilivigni/cockpit,Scribery/cockpit,FireDrunk/cockpit,michalskrivanek/cockpit,maxamillion/cockpit,garrett/cockpit,maxamillion/cockpit,vbatts/cockpit,evol262/cockpit,maxamillion/cockpit,larsu/cockpit,darioajr/cockpit,netzvieh/cockpit,mareklibra/cockpit,cockpituous/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mareklibra/cockpit,firebitsbr/cockpit,larskarlitski/cockpit,kkaempf/cockpit,zigitax/cockpit,xhad/cockpit,mvollmer/cockpit,larskarlitski/cockpit,SotolitoLabs/cockpit,kkaempf/cockpit,larsu/cockpit,deryni/cockpit,larskarlitski/cockpit,vanloswang/cockpit,moraleslazaro/cockpit,michalskrivanek/cockpit,martinpitt/cockpit,mvollmer/cockpit,nmav/cockpit,sgallagher/cockpit,sgallagher/cockpit,nmav/cockpit,harishanand95/cockpit,zigitax/cockpit,deryni/cockpit,moolitayer/cockpit,vbatts/cockpit,moolitayer/cockpit,cockpituous/cockpit,xhad/cockpit,Thermionix/cockpit,Scribery/cockpit,martinpitt/cockpit,jscotka/cockpit,jscotka/cockpit,larsu/cockpit,SotolitoLabs/cockpit,moolitayer/cockpit,netzvieh/cockpit,moraleslazaro/cockpit,vbatts/cockpit,andreasn/cockpit,Scribery/cockpit,FireDrunk/cockpit,Armstrong1992/cockpit,Armstrong1992/cockpit,moraleslazaro/cockpit,arilivigni/cockpit,fridex/cockpit,FireDrunk/cockpit,xhad/cockpit,haiyangd/cockpit_view,petervo/cockpit,stefwalter/cockpit,denysvitali/cockpit,dperpeet/cockpit,mareklibra/cockpit,mvollmer/cockpit,netzvieh/cockpit,michalskrivanek/cockpit,denysvitali/cockpit,FireDrunk/cockpit,sub-mod/cockpit,andreasn/cockpit,petervo/cockpit,harishanand95/cockpit,matobet/cockpit,mareklibra/cockpit,larsu/cockpit,sgallagher/cockpit,matobet/cockpit,sub-mod/cockpit,SotolitoLabs/cockpit,jscotka/cockpit,SotolitoLabs/cockpit,darioajr/cockpit,kkaempf/cockpit,sgallagher/cockpit,dperpeet/cockpit,denysvitali/cockpit,moolitayer/cockpit,darioajr/cockpit,matobet/cockpit,firebitsbr/cockpit,sub-mod/cockpit,michalskrivanek/cockpit,xhad/cockpit,mareklibra/cockpit,vbatts/cockpit,larsu/cockpit,cockpituous/cockpit,zigitax/cockpit,Thermionix/cockpit,stefwalter/cockpit,stefwalter/cockpit,arilivigni/cockpit,xhad/cockpit,denysvitali/cockpit,deryni/cockpit,vanloswang/cockpit,michalskrivanek/cockpit,vanloswang/cockpit,Scribery/cockpit,larskarlitski/cockpit,deryni/cockpit,fridex/cockpit,Scribery/cockpit,moraleslazaro/cockpit,Armstrong1992/cockpit,petervo/cockpit,xhad/cockpit,andreasn/cockpit,sgallagher/cockpit,cockpit-project/cockpit,Thermionix/cockpit,haiyangd/cockpit_view,darioajr/cockpit,denysvitali/cockpit,larsu/cockpit,evol262/cockpit,netzvieh/cockpit,martinpitt/cockpit,maxamillion/cockpit,petervo/cockpit,matobet/cockpit,jscotka/cockpit,fridex/cockpit,Armstrong1992/cockpit,moolitayer/cockpit,moraleslazaro/cockpit,petervo/cockpit,andreasn/cockpit,haiyangd/cockpit_view,maxamillion/cockpit,sub-mod/cockpit,Scribery/cockpit,vbatts/cockpit,mvollmer/cockpit,arilivigni/cockpit,matobet/cockpit,kkaempf/cockpit,petervo/cockpit,Thermionix/cockpit,dperpeet/cockpit,nmav/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,cockpit-project/cockpit,haiyangd/cockpit_view,xhad/cockpit,harishanand95/cockpit,arilivigni/cockpit,petervo/cockpit,cockpituous/cockpit,matobet/cockpit,fridex/cockpit,firebitsbr/cockpit,vanloswang/cockpit,FireDrunk/cockpit,larskarlitski/cockpit,kkaempf/cockpit,michalskrivanek/cockpit,arilivigni/cockpit,evol262/cockpit,andreasn/cockpit,Scribery/cockpit,cockpituous/cockpit,darioajr/cockpit,deryni/cockpit,deryni/cockpit,sgallagher/cockpit,martinpitt/cockpit,netzvieh/cockpit,Thermionix/cockpit,larskarlitski/cockpit,evol262/cockpit,haiyangd/cockpit_view,firebitsbr/cockpit,dperpeet/cockpit,fridex/cockpit,deryni/cockpit,harishanand95/cockpit,jscotka/cockpit,harishanand95/cockpit,harishanand95/cockpit,larskarlitski/cockpit,sub-mod/cockpit,evol262/cockpit,stefwalter/cockpit,Thermionix/cockpit,andreasn/cockpit,zigitax/cockpit,cockpituous/cockpit,dperpeet/cockpit,evol262/cockpit,cockpit-project/cockpit,sgallagher/cockpit,harishanand95/cockpit,garrett/cockpit,dperpeet/cockpit,fridex/cockpit,stefwalter/cockpit,Armstrong1992/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,nmav/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,michalskrivanek/cockpit,cockpit-project/cockpit,garrett/cockpit,jscotka/cockpit,arilivigni/cockpit,SotolitoLabs/cockpit,garrett/cockpit,nmav/cockpit,netzvieh/cockpit,andreasn/cockpit,dperpeet/cockpit,vanloswang/cockpit,SotolitoLabs/cockpit,zigitax/cockpit,firebitsbr/cockpit