Move bus type and related APIs to a separate file bus.c.
This is a first step in breaking up qdev.c into more manageable chunks.
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[AF: Rebased onto osdep.h]
Signed-off-by: Andreas Färber <afaerber@suse.de>
[PMM: added bus.o to link line for test-qdev-global-props]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
