context: new switching api, vgic/context: fix to guest 1 launching failure

1. Context: new api, context_switchto(vmid) and context_perform_switch() instead of context_switch_to_next_guest()
2. VGIC/Context: context now saves/restores PL1 vbar and initializes VGIC state for a guest launches first time to avoid crash due to invalid initial state of guest 1