vfio/pci: Fix instance_size of VFIO_PCI_BASE

vfio/pci: Fix instance_size of VFIO_PCI_BASE

Currently the final instance_size of VFIO_PCI_BASE is sizeof(PCIDevice).
It should be sizeof(VFIOPCIDevice), VFIO_PCI uses same structure as
base class VFIO_PCI_BASE, so no need to set its instance_size explicitly.

This isn't catastrophic only because VFIO_PCI_BASE is an abstract class.

Fixes: d4e392d0a99b ("vfio: add vfio-pci-base class")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250611024228.423666-1-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>