qemu: don't modify domain on failed blockiotune

If you have a qemu build that lacks the blockio tune monitor command,
then this command:

$ virsh blkdeviotune rhel6u2 hda --total_bytes_sec 1000
error: Unable to change block I/O throttle
error: internal error Unexpected error

fails as expected (well, the error message is lousy), but the next
dumpxml shows that the domain was modified anyway.  Worse, that means
if you save the domain then restore it, the restore will likely fail
due to throttling being unsupported, even though no throttling should
even be active because the monitor command failed in the first place.

* src/qemu/qemu_driver.c (qemuDomainSetBlockIoTune): Check for
error before making modification permanent.

src/qemu/qemu_driver.c

@@ -12011,9 +12011,9 @@ qemuDomainSetBlockIoTune(virDomainPtr dom,
         qemuDomainObjEnterMonitorWithDriver(driver, vm);
         ret = qemuMonitorSetBlockIoThrottle(priv->mon, device, &info);
         qemuDomainObjExitMonitorWithDriver(driver, vm);
-        vm->def->disks[idx]->blkdeviotune = info;
         if (ret < 0)
             goto endjob;
+        vm->def->disks[idx]->blkdeviotune = info;
     }
 
     if (flags & VIR_DOMAIN_AFFECT_CONFIG) {