hmp: add info iothreads command

Make "info iothreads" available on the HMP monitor.

For example, the results are as follows when executing qemu
command with "-object iothread,id=iothread-1 -object
iothread,id=iothread-2".
(qemu) info iothreads
iothread-1: thread_id=123
iothread-2: thread_id=456

Signed-off-by: Ting Wang <kathy.wangting@huawei.com>
Message-Id: <1435306033-58372-1-git-send-email-kathy.wangting@huawei.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Amos Jianjun Kong <kongjianjun@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>