net: use qemu_send_packet_raw() in qemu_announce_self()

Use qemu_send_packet_raw to send gratuitous arp. This will ensure that
vnet header is handled properly.

Also, avoid sending the gratuitous packet to the guest. There doesn't
appear to be any reason for doing that and the code will currently just
crash if the NIC is not associated with a vlan.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
