Adding a proper receive_iov function to the net dump module.
This will make it easier to support the dump filter feature for
the -netdev option in later patches.

Reviewed-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>