https://bugzilla.redhat.com/show_bug.cgi?id=1356937

Add the definitions to allow for viewing/setting cgroup period and quota
limits for IOThreads.

This is similar to the work done for emulator quota and period by
commit ids 'b65dafa8' and 'e051c482'.

Being able to view/set the IOThread specific values is related to more
recent changes adding global period (commmit id '4d92d58f') and global
quota (commit id '55ecdae0') definitions and qemu support (commit id
'4e17ff79' and 'fbcbd1b2'). With a global setting though, if somehow
the IOThread value in the cgroup hierarchy was set "outside of libvirt"
to a value that is incompatible with the global value.

Allowing control over IOThread specific values provides the capability
to alter the IOThread values as necessary.