examples/vhost: handle broadcast packet

examples/vhost: handle broadcast packet

Every time I do a VM2VM iperf test with vhost example, I have to set
the arp table manually, as vhost-switch just ignores the broadcast
packet, leaving the ARP request not served.

Here we do a transmit a broadcast packet (such as ARP request) to
every vhost device, as well as the physical port, to fix above
arp table issue.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>