qemu: ensure domain event thread is always stopped

qemu: ensure domain event thread is always stopped

In previous commit:

  commit e6afacb0feabd9bf58331aa0e5259a35378be74e
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Feb 12 12:26:11 2020 +0000

    qemu: start/stop an event loop thread for domains

A bogus comment was added claiming we didn't need to shutdown the
event thread in the qemuProcessStop method, because this would be
done in the monitor EOF callback. This was wrong because the EOF
callback only runs in the case of a QEMU crash or a guest initiated
clean shutdown & poweroff.  In the case where the libvirt admin
calls virDomainDestroy, the EOF callback never fires because we
have already unregistered the event callbacks. We must thus always
attempt to stop the event thread in qemuProcessStop.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reported-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>