sysbus: Make devices spawnable via -device

Now that we can properly map sysbus devices that haven't been connected to
something forcefully by C code, we can allow the -device command line option
to spawn them.

For machines that don't implement dynamic sysbus assignment in their board
files we add a new bool "has_dynamic_sysbus" to the machine class.
When that property is false (default), we bail out when we see dynamically
spawned sysbus devices, like we did before.

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