CVE-2026-64475: vfio/pci: Release the VGA arbiter client on register_device() failure

In the Linux kernel, the following vulnerability has been resolved:

vfio/pci: Release the VGA arbiter client on registerdevice() failure

The re-order in the Fixes commit below displaced vfiopcivgainit() as the last failure point of what is now vfiopcicoreregisterdevice() without introducing an unwind for the VGA arbiter registration.

In current kernels this is mostly benign because vfiopcisetdecode() only uses pcidev state, but the original failure path could leave a callback with a freed vdev cookie. The stale registration also becomes unsafe again once the callback follows drvdata to the vfio device.

Add the required VGA unwind callout.