net: fix vnet_hdr bustage with slirp
slirp has started using VLANClientState::opaque and this has caused the
kvm specific tap_has_vnet_hdr() hack to break because we blindly use
this opaque pointer even if it is not a tap client.
Add yet another hack to check that we're actually getting called with a
tap client.
[Needed on stable-0.11 too]
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
(cherry picked from commit
8494e711d600ccac0d6ffaae54d41b0ce5fd28ba)