vhost: handle request to send RARP

vhost: handle request to send RARP

While in former patch we enabled GUEST_ANNOUNCE feature, so that the
guest OS will broadcast a GARP message after migration to notify the
switch about the new location of migrated VM, the thing is that
GUEST_ANNOUNCE is enabled since kernel v3.5 only. For older kernel,
VHOST_USER_SEND_RARP request comes to rescue.

The payload of this new request is the mac address of the migrated VM,
with that, we could construct a RARP message, and then broadcast it
to host interfaces.

That's how this patch works:

- list all interfaces, with the help of SIOCGIFCONF ioctl command

- construct an RARP message and broadcast it

Cc: Thibaut Collet <thibaut.collet@6wind.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>