virsh: Check whether enough arguments was passed to iothreadset
virsh: Check whether enough arguments was passed to iothreadset

Virsh has iothreadset command which allows setting various
attributes of IOThreads. However, when the command is called
without any arguments (besides domain and IOThread IDs), then
@params stays NULL and is passed to virDomainSetIOThreadParams()
which produces rather user unfriendly error message:

  error: params in virDomainSetIOThreadParams must not be NULL

Introduce a check and produce better error message.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Claudio Fontana <b69352cd5992c270120dd2b68be30033a38f7beb@suse.de>

zippy2/libvirt,zippy2/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt