vl.c: Improve message when no default machine is found

Improve the clarity of the message QEMU prints when the user
doesn't specify a machine model to use and there is no default.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.a@redhat.com>

vl.c

@@ -3929,7 +3929,8 @@ int main(int argc, char **argv, char **envp)
 #endif
 
     if (machine_class == NULL) {
-        fprintf(stderr, "No machine found.\n");
+        fprintf(stderr, "No machine specified, and there is no default.\n"
+                "Use -machine help to list supported machines!\n");
         exit(1);
     }