slirp: Fix restricted mode This aligns the code to what the documentation claims: Allow everything but requests that would have to be routed outside of the virtual LAN. So we need to drop the unneeded IP-level filter, allow TFTP requests, and add the missing protocol-level filter to ICMP. CC: Gleb Natapov <gleb@redhat.com> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>