qemu_cgroup: Introduce cgroup functions for IOThreads

qemu_cgroup: Introduce cgroup functions for IOThreads

In order to support cpuset setting, introduce qemuSetupCgroupIOThreadsPin
and qemuSetupCgroupForIOThreads to mimic the existing Vcpu API's.

These will support having an 'iotrhreadpin' element in the 'cpuset' in
order to pin named IOThreads to specific CPU's. The IOThread pin names
will follow the IOThread naming scheme starting at 1 (eg "iothread1")
up through an including the def->iothreads value.