Return Futures from Controller Methods

[oberien]

Action-performing methods should return a future which resolves as soon as the performed action is confirmed by qemu / the GA.

• io_attach: Resolves when {return: {}} is returned by qemu. When waking up from suspend it resolves after attaching the devices. If all devices are already attached, an instataneously resolving future will be returned.
• io_force_attach: ~
• io_detach: Resolves when DEVICE_DELETED is returned by qemu as often as the number of devices to be disconnected.
• action: Resolves based on the previous action taken.
• suspend: Resolves when SUSPEND is returned by qemu.
• shutdown: Resolves when SHUTDOWN or SUSPEND_DISK is returned by qemu.

[main--]

Consider doing this a little differently: Instead of just dumping naked device add commands into the stream, implement what is essentially an RPC wrapper where you add a single device asynchronously. It also needs to have a lock so we don't have multiple concurrent calls (or else we have no clue what call a return belongs with (?)). Then io attach/detach just calls that for every device.