In qdev, each bus in practice identified an abstract superclass, but
this was mostly hidden.  In QOM, instead, these abstract classes are
explicit so we can move bus properties there.

All bus property walks are removed, and all device property walks
are changed to look along the class hierarchy instead.

We would have duplicates if class A defines some properties and its
subclass B does not define any, because class_b->props will be
left equal to class_a->props.

The solution here is to reintroduce the class_base_init TypeInfo
callback, that was present in one of the early QOM versions but
removed (on my request...) before committing.

This breaks global bus properties, an obscure feature when used
with the command-line which is actually useful and used when used by
backwards-compatible machine types.  So this patch also adjusts the
global bus properties in hw/pc_piix.c to refer to the abstract class.

Globals and other properties must be modified in the same patch to
avoid complications related to initialization ordering.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>