qemu: update configuration file

* src/qemu/qemu.conf: Add blkio controller into qemu.conf.

src/qemu/qemu.conf

@@ -156,18 +156,19 @@
 #  - 'cpu' - use for schedular tunables
 #  - 'devices' - use for device whitelisting
 #  - 'memory' - use for memory tunables
+#  - 'blkio' - use for block devices I/O tunables
 #
 # NB, even if configured here, they won't be used unless
 # the administrator has mounted cgroups, e.g.:
 #
 #  mkdir /dev/cgroup
-#  mount -t cgroup -o devices,cpu,memory none /dev/cgroup
+#  mount -t cgroup -o devices,cpu,memory,blkio none /dev/cgroup
 #
 # They can be mounted anywhere, and different controllers
 # can be mounted in different locations. libvirt will detect
 # where they are located.
 #
-# cgroup_controllers = [ "cpu", "devices", "memory" ]
+# cgroup_controllers = [ "cpu", "devices", "memory", "blkio" ]
 
 # This is the basic set of devices allowed / required by
 # all virtual machines.