sysbus: Add new platform bus helper device

We need to support spawning of sysbus devices dynamically via the command line.
The easiest way to represent these dynamically spawned devices in the guest's
memory and IRQ layout is by preallocating some space for dynamic sysbus devices.

This is what the "platform bus" device does. It is a sysbus device that exports
a configurably sized MMIO region and a configurable number of IRQ lines. When
this device encounters sysbus devices that have been dynamically created and not
manually wired up, it dynamically connects them to its own pool of resources.

The machine model can then loop through all of these devices and create a guest
configuration (device tree) to make them visible to the guest.

Signed-off-by: Alexander Graf <agraf@suse.de>
