net/slirp: fix the IPv6 prefix length error message

Reword and add a missing parentheses at the end of the
error message.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/slirp.c

@@ -498,7 +498,8 @@ static int net_slirp_init(NetClientState *peer, const char *model,
     }
     if (vprefix6_len < 0 || vprefix6_len > 126) {
         error_setg(errp,
-                   "Invalid prefix provided (prefix len must be in range 0-126");
+                   "Invalid IPv6 prefix provided "
+                   "(IPv6 prefix length must be between 0 and 126)");
         return -1;
     }