qerror: Clean up QERR_ macros to expand into a single string

These macros expand into error class enumeration constant, comma,
string.  Unclean.  Has been that way since commit 13f59ae.

The error class is always ERROR_CLASS_GENERIC_ERROR since the previous
commit.

Clean up as follows:

* Prepend every use of a QERR_ macro by ERROR_CLASS_GENERIC_ERROR, and
  delete it from the QERR_ macro.  No change after preprocessing.

* Rewrite error_set(ERROR_CLASS_GENERIC_ERROR, ...) into
  error_setg(...).  Again, no change after preprocessing.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>

backends/rng.c

@@ -57,7 +57,7 @@ static void rng_backend_prop_set_opened(Object *obj, bool value, Error **errp)
     }
 
     if (!value && s->opened) {
-        error_set(errp, QERR_PERMISSION_DENIED);
+        error_setg(errp, QERR_PERMISSION_DENIED);
         return;
     }