Commit d03e1b3 ("update submodule and patches to 7.2.0") argued that
slirp is not explicitly supported in PVE, but that is not true. In
qemu-server, user networking is supported (via CLI/API) when no bridge
is set on a virtual NIC. So slirp needs to stay to keep such NICs
working.
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>