vfio: add vfio-pci-base class

vfio: add vfio-pci-base class

Split out parts of TYPE_VFIO_PCI into a base TYPE_VFIO_PCI_BASE,
although we have not yet introduced another subclass, so all the
properties have remained in TYPE_VFIO_PCI.

Note that currently there is no need for additional data for
TYPE_VFIO_PCI, so it shares the same C struct type as
TYPE_VFIO_PCI_BASE, VFIOPCIDevice.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250507152020.1254632-14-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>