https://bugzilla.redhat.com/show_bug.cgi?id=1101574

Add an option 'iothreadpin' to the <cpuset> to allow for setting the
CPU affinity for each IOThread.

The iothreadspin will mimic the vcpupin with respect to being able to
assign each iothread to a specific CPU, although iothreads ids start
at 1 while vcpu ids start at 0. This matches the iothread naming scheme.