virtio-pci: fix bus master work around on load

virtio-pci: fix bus master work around on load

Commit c81131db15dd1844d0db1d51f3cd7a105cfd2cf3
detects old guests by comparing virtio and
PCI status. It attempts to do this on load,
as well, but load_config callback in a binding
is invoked too early and so the virtio status
isn't set yet.

We could add yet another callback to the
binding, to invoke after load, but it
seems easier to reuse the existing vmstate
callback.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>