slirp: fill error when failing to initialize user network

With "-netdev user,id=net0,dns=1.2.3.4"
error was:
qemu-system-i386: -netdev user,id=net0,dns=1.2.3.4: Device 'user' could not be initialized

Error is now:
qemu-system-i386: -netdev user,id=net0,dns=1.2.3.4: DNS doesn't belong to network

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
