conf: Add missing return in conf_nat(), fix --map-guest-addr none

As reported by somebody on IRC:

  $ pasta --map-guest-addr none
  Invalid address to remap to host: none

that's because once we parsed "none", we try to parse it as an address
as well. But we already handled it, so stop once we're done.

Fixes: e813a4df7d ("conf: Allow address remapped to host to be configured")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

conf.c

@@ -1272,6 +1272,8 @@ static void conf_nat(const char *arg, struct in_addr *addr4,
 		*addr6 = in6addr_any;
 		if (no_map_gw)
 			*no_map_gw = 1;
+
+		return;
 	}
 
 	if (inet_pton(AF_INET6, arg, addr6)	&&