https://bugzilla.redhat.com/show_bug.cgi?id=1161617

Add command to allow adding and removing IOThreads from the domain including
the configuration and live domain.

$ virsh iothreadadd --help
  NAME
    iothreadadd - add an IOThread to the guest domain

  SYNOPSIS
    iothreadadd <domain> <id> [--config] [--live] [--current]

  DESCRIPTION
    Add an IOThread to the guest domain.

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    [--id] <number>  iothread for the new IOThread
    --config         affect next boot
    --live           affect running domain
    --current        affect current domain

$ virsh iothreaddel --help
  NAME
    iothreaddel - delete an IOThread from the guest domain

  SYNOPSIS
    iothreaddel <domain> <id> [--config] [--live] [--current]

  DESCRIPTION
    Delete an IOThread from the guest domain.

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    [--id] <number>  iothread_id for the IOThread to delete
    --config         affect next boot
    --live           affect running domain
    --current        affect current domain

Assuming a running $dom with multiple IOThreads assigned and that
that the $dom has disks assigned to IOThread 1 and IOThread 2:

$ virsh iothreadinfo $dom
 IOThread ID     CPU Affinity
 ---------------------------------------------------
  1               2
  2               3
  3               0-1

$ virsh iothreadadd $dom 1
error: invalid argument: an IOThread is already using iothread_id '1' in iothreadpids

$ virsh iothreadadd $dom 1 --config
error: invalid argument: an IOThread is already using iothread_id '1' in persistent iothreadids

$ virsh iothreadadd $dom 4
$ virsh iothreadinfo $dom
 IOThread ID     CPU Affinity
 ---------------------------------------------------
  1               2
  2               3
  3               0-1
  4               0-3

$ virsh iothreadinfo $dom --config
 IOThread ID     CPU Affinity
 ---------------------------------------------------
  1               2
  2               3
  3               0-1

$ virsh iothreadadd $dom 4 --config
$ virsh iothreadinfo $dom --config
 IOThread ID     CPU Affinity
  ---------------------------------------------------
    1               2
    2               3
    3               0-1
    4               0-3

Assuming the same original configuration

$ virsh iothreaddel $dom 1
error: invalid argument: cannot remove IOThread 1 since it is being used by disk 'vde'

$ virsh iothreaddel $dom 3

$ virsh iothreadinfo $dom
 IOThread ID     CPU Affinity
 ---------------------------------------------------
  1               2
  2               3

$ virsh iothreadinfo $dom --config
 IOThread ID     CPU Affinity
 ---------------------------------------------------
  1               2
  2               3
  3               0-1