virtio: new post_load hook

virtio: new post_load hook

Post load hook in virtio vmsd is called early while device is processed,
and when VirtIODevice core isn't fully initialized.  Most device
specific code isn't ready to deal with a device in such state, and
behaves weirdly.

Add a new post_load hook in a device class instead.  Devices should use
this unless they specifically want to verify the migration stream as
it's processed, e.g. for bounds checking.

Cc: [email protected]
Suggested-by: "Dr. David Alan Gilbert" <[email protected]>
Cc: Mikhail Sennikovsky <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Jason Wang <[email protected]>