virtio-gpu: fix v2 migration

virtio-gpu: fix v2 migration

Commit dfcf74fa ("virtio-gpu: fix scanout migration post-load") broke
forward/backward version migration. Versioning of nested VMSD structures
is not straightforward, as the wire format doesn't have nested
structures versions. Introduce x-scanout-vmstate-version and a field
test to save/load appropriately according to the machine version.

Fixes: dfcf74fa ("virtio-gpu: fix scanout migration post-load")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
[fixed long lines]
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit 40a23ef643664b5c1021a9789f9d680b6294fb50)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>