virtio-net: Switch to using announce timer

virtio-net: Switch to using announce timer

Switch virtio's self announcement to use the AnnounceTimer.
It keeps it's own AnnounceTimer (per device), and starts running it
using a migration post-load and a virtual clock; that way the
announce happens once the guest is actually running.
The timer uses the migration parameters to set the timing of
the repeats.

Based on earlier patches by myself and
 Vladislav Yasevich <vyasevic@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>