net: virtio-net and vmxnet3 use offloading API

With this patch, virtio-net and vmxnet3 frontends make
use of the qemu_peer_* API for backend offloadings manipulations,
instead of calling TAP-specific functions directly.
We also remove the existing checks which prevent those frontends
from using offloadings with backends different from TAP (e.g. netmap).

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
