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>
diff --git a/block.c b/block.c
index dd4f58d..3bdc995 100644
--- a/block.c
+++ b/block.c
@@ -2698,7 +2698,7 @@
             error_setg(errp, "Node '%s' is not encrypted",
                       bdrv_get_device_or_node_name(bs));
         } else if (bdrv_set_key(bs, key) < 0) {
-            error_set(errp, QERR_INVALID_PASSWORD);
+            error_setg(errp, QERR_INVALID_PASSWORD);
         }
     } else {
         if (bdrv_key_required(bs)) {