balloon: Eliminate silly QERR_ macros

The QERR_ macros are leftovers from the days of "rich" error objects.
They're used with error_set() and qerror_report(), and expand into the
first *two* arguments.  This trickiness has become pointless.  Clean
up the balloon ones.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/balloon.c b/balloon.c
index 728bb70..dea19a4 100644
--- a/balloon.c
+++ b/balloon.c
@@ -39,11 +39,13 @@
 static bool have_ballon(Error **errp)
 {
     if (kvm_enabled() && !kvm_has_sync_mmu()) {
-        error_set(errp, QERR_KVM_MISSING_CAP, "synchronous MMU", "balloon");
+        error_set(errp, ERROR_CLASS_KVM_MISSING_CAP,
+                  "Using KVM without synchronous MMU, balloon unavailable");
         return false;
     }
     if (!balloon_event_fn) {
-        error_set(errp, QERR_DEVICE_NOT_ACTIVE, "balloon");
+        error_set(errp, ERROR_CLASS_DEVICE_NOT_ACTIVE,
+                  "No balloon device has been activated");
         return false;
     }
     return true;