kvm tools: virtio-net has to open tap device before vhost-net init.

Enabling vhost-net encounted an error:
  Fatal: VHOST_NET_SET_BACKEND failed 88

The reason is that vhost-net requires tap_fd for VHOST_NET_SET_BACKEND,
however tap_fd is opened after VIRTIO_CONFIG_S_DRIVER_OK. Because the
initialization needs to know the guest features, I suppose the initialization
could be moved to set_guest_features(). Therefore, initialization can be
finished before status VIRTIO_CONFIG_S_DRIVER_OK, and tap_fd can be set
before vhost-net sets backend.

Signed-off-by: Ying-Shiuan Pan <yspan@itri.org.tw>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
kvm tools: vhost-net: setup mergeable rx buffers feature

After features negotiation, kvmtool should tell vhost-net that he's
using mergeable rx buffers.

Signed-off-by: Ying-Shiuan Pan <yspan@itri.org.tw>
Signed-off-by: Pekka Enberg <penberg@kernel.org>