qemu: handle multicast overflow on macvtap NIC_RX_FILTER_CHANGED

qemu: handle multicast overflow on macvtap NIC_RX_FILTER_CHANGED

Guest network devices can set 'overflow' when there are a number of multicast
ips configured. For virtio_net, the limit is only 64. In this case, the list
of mac addresses is empty and the 'overflow' condition is set. Thus, the guest
will currently receive no multicast traffic in this state.

When 'overflow' is set in the guest, let's turn this into ALLMULTI on the host.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>