qdev/compat: compat property infrastructure.

qdev/compat: compat property infrastructure.

This add support for switching devices into a compatibility mode
using device properties.  Machine types can have a list of properties
for specific devices attached to allow the easy creation of machine
types compatible to older qemu versions.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>