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/qemu-img.c b/qemu-img.c
index ea964fe..6d1b136 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -739,7 +739,7 @@
     if (base) {
         base_bs = bdrv_find_backing_image(bs, base);
         if (!base_bs) {
-            error_set(&local_err, QERR_BASE_NOT_FOUND, base);
+            error_setg(&local_err, QERR_BASE_NOT_FOUND, base);
             goto done;
         }
     } else {