qemuAgentGetUsers: Fix return value on success

Return 0 on success to match the documentation. The callers only check
for negative values.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_agent.c

@@ -2399,7 +2399,7 @@ qemuAgentGetUsers(qemuAgentPtr agent,
             return -1;
     }
 
-    return ndata;
+    return 0;
 }
 
 /* Returns: 0 on success