vhost: reserve few more space for future extension

vhost: reserve few more space for future extension

"virtio_net_device_ops" is the only left open struct that an application
can access, therefore, it's the only place that might introduce potential
ABI break in future for extension.

So, do some reservation for it. 5 should be pretty enough, considering
that we have barely touched it for a long while. Another reason to
choose 5 is for cache alignment: 5 makes the struct 64 bytes for 64 bit
machine.

With this, it's confidence to say that we might be able to be free from
the ABI violation forever.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>