iothread: fix crash with invalid properties

-object iothread,id=foo,? will crash qemu:

qemu-system-x86_64:qemu-thread-posix.c:128: qemu_cond_destroy: Assertion `cond->initialized' failed.

Use thread_id != -1 to check if iothread_complete() finished
successfully and the mutex/cond have been initialized.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180821100716.13803-1-marcandre.lureau@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>