qemu: Improve error if setmem fails for lacking of balloon support

"cannot set memory of an active domain" is misleading, it sounds
like setting memory of active domain is not supported.

src/qemu/qemu_driver.c

@@ -2037,7 +2037,8 @@ static int qemudDomainSetMemoryFlags(virDomainPtr dom, unsigned long newmem,
             /* Lack of balloon support is a fatal error */
             if (r == 0) {
                 qemuReportError(VIR_ERR_OPERATION_INVALID, "%s",
-                                _("cannot set memory of an active domain"));
+                                _("Unable to change memory of active domain without "
+                                  "the balloon device and guest OS balloon driver"));
                 goto endjob;
             }
         }