vhost: remove unnecessary fields

vhost: remove unnecessary fields

The "reserved" field in virtio_net and vhost_virtqueue struct is not
necessary any more. We now expose virtio_net device with a number "vid".

This patch also removes the "priv" field: all fields are priviate now:
application can't access it now. The only way that we could still access
it is to expose it by a function, but I doubt that's needed or worthwhile.

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>