iothread: Make iothread_stop() idempotent

Currently, iothread_stop_all() makes all iothread objects unsafe
to be destroyed, because qemu_thread_join() ends up being called
twice.

To fix this, make iothread_stop() idempotent by checking
thread->stopped.

Fixes the following crash:

  qemu-system-x86_64 -object iothread,id=iothread0 -monitor stdio -display none
  QEMU 2.10.50 monitor - type 'help' for more information
  (qemu) quit
  qemu: qemu_thread_join: No such process
  Aborted (core dumped)

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170926130028.12471-1-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
