virtio-pci: fill VirtIOPCIRegions early.

Initialize the modern bar and the VirtIOPCIRegion fields early, in
realize.  Also add a size field to VirtIOPCIRegion and variables for
pci bars to VirtIOPCIProxy.

This allows virtio-pci subclasses to change things before the
device_plugged callback applies them.  virtio-vga will use that to
arrange regions in a way that virtio-vga is compatible to both stdvga
(in vga mode) and virtio-gpu-pci (in pci mode).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
