dom0+vm/qrexec-services: pass remote domain via env variable not argument

Most qrexec services doesn't use remote domain name, as policy is enforced
earlier. So pass it in way that will allow use of generic command as qrexec
service.

qubes_rpc/qfile-unpacker.c

@@ -51,13 +51,19 @@ int main(int argc, char ** argv)
 	char *incoming_dir;
 	int pipefds[2];
 	int uid;
+	char *remote_domain;
 
 	pipe(pipefds);
 
 	uid = prepare_creds_return_uid("user");
 
+	remote_domain = getenv("QREXEC_REMOTE_DOMAIN");
+	if (!remote_domain) {
+		gui_fatal("Cannot get remote domain name");
+		exit(1);
+	}
 	mkdir(INCOMING_DIR_ROOT, 0700);
-	asprintf(&incoming_dir, "%s/from-%s", INCOMING_DIR_ROOT, argv[1]);
+	asprintf(&incoming_dir, "%s/from-%s", INCOMING_DIR_ROOT, remote_domain);
 	mkdir(incoming_dir, 0700);
 	if (chdir(incoming_dir))
 		gui_fatal("Error chdir to %s", incoming_dir);