Fixes for Linux Radeon with 440FX guests

The DRM and Radeon drivers in Linux assume that there's always a parent device to the GPU.  We can break this assumption easily with either the 440FX or Q35 QEMU machine modules by attaching the GPU to the root bus.  This has been one of the problems drawing users to more complicated Q35 models which more accurately reflect the host hardware.  We can also fix the driver to avoid such assumptions:

[PATCH] drm: Test for PCI root bus to avoid NULL pointer dereference

[PATCH] radeon: Test for PCI root bus before assuming bus->self