Upstream slirp4netns patches
Patches from https://github.com/rootless-containers/slirp4netns/tree/dda4568b9357df5170a1e5b2bf3e0c35d38c2559/qemu_patches
- add slirp_initx(SlirpConfig *, SlirpCb *, void *)
- allow custom MTU
- add disable_host_loopback (prohibit connections to 127.0.0.1)