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 register_device() failure The re-order in the Fixes commit below displaced vfio_pci_vga_init() as the last failure point of what is now vfio_pci_core_register_device() without introducing an unwind for the VGA arbiter registration. In current kernels this is mostly benign because vfio_pci_set_decode() only uses pci_dev 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.