slirp: Add explanation for hostfwd parsing failure

e.g.
./x86_64-softmmu/qemu-system-x86_64 -nographic -netdev 'user,id=vnet,hostfwd=:555.0.0.0:0-:22'
qemu-system-x86_64: -netdev user,id=vnet,hostfwd=:555.0.0.0:0-:22: Invalid host forwarding rule ':555.0.0.0:0-:22' (Bad host address)

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
