qemu: Add check for invalid iothread_id in qemuDomainChgIOThread

qemu: Add check for invalid iothread_id in qemuDomainChgIOThread

https://bugzilla.redhat.com/show_bug.cgi?id=1251886

Since iothread_id == 0 is an invalid value for QEMU let's point
that out specifically.  For the IOThreadDel code, the failure would
have ended up being a failure to find the IOThread ID; however, for
the IOThreadAdd code - an IOThread 0 was added and that isn't good.

It seems during many reviews/edits to the code the check for
iothread_id = 0 being invalid was lost - it could have originally
been in the API code, but requested to be moved - I cannot remember.