Description of problem:
iothreadpin will pin one of libvirtd thread with qemu 1.5

Version-Release number of selected component (if applicable):
qemu-kvm-1.5.3-97.el7.x86_64
libvirt-1.2.17-3.el7.x86_64

How reproducible:
100%

Steps to Reproduce:

1. prepare a guest with iothreadpin:

# virsh dumpxml test4 |grep iothread
  <iothreads>1</iothreads>
  <iothreadids>
    <iothread id='1'/>
  </iothreadids>
    <iothreadpin iothread='1' cpuset='2'/>


2. start it and check libvirt set in cgroup

# ll /proc/9580/task/
total 0
dr-xr-xr-x. 6 qemu qemu 0 Jul 27 10:31 9580    <----emulator
dr-xr-xr-x. 6 qemu qemu 0 Jul 27 10:32 9589    <----vcpu0
dr-xr-xr-x. 6 qemu qemu 0 Jul 27 10:32 9591    <----vcpu1
dr-xr-xr-x. 6 qemu qemu 0 Jul 27 10:32 9600    <----???

# cat /sys/fs/cgroup/cpuset/machine.slice/machine-qemu\\x2dtest4.scope/iothread1/cpuset.cpus
2

# cat /sys/fs/cgroup/cpuset/machine.slice/machine-qemu\\x2dtest4.scope/iothread1/tasks 
4402

# ps -eLf |grep 4402
root      4391     1  4402  0   16 10:28 ?        00:00:00 /usr/sbin/libvirtd
root     16347 19012 16347  0    1 11:15 pts/0    00:00:00 grep --color=auto 4402

3. check taskset
# taskset --cpu-list -p 4402
pid 4402's current affinity list: 2


4. Then try to change the iothreadpin and recheck the result:

# virsh iothreadpin test4 1 3 

# taskset --cpu-list -p 4402
pid 4402's current affinity list: 3

# cat /sys/fs/cgroup/cpuset/machine.slice/machine-qemu\\x2dtest4.scope/iothread1/cpuset.cpus
3


Actual results:
iothreadpin will pin one of libvirtd thread with qemu 1.5

Expected results:
forbid use iotreadpin with qemu which not support iothread

Additional info:

This is because we cannot get iothread_id and it's value is 0, and then we call
sched_setaffinity() and attach thread in cgroup, but when the pid/task is 0, these functions will attach the current thread, you can check kernel doc Documentation/cgroups/cgroups.txt:

You can attach the current shell task by echoing 0:

# echo 0 > tasks

and manual of sched_setaffinity(2):

Specifying pid as 0 will set the attribute for the calling thread

So i think we should also forbid set 0 as a pid when call these functions.