Under the right circumstances, xenconsoled will corrupt its internal

Under the right circumstances, xenconsoled will corrupt its internal
list of domains causing a SEGV.  This is usually characterized by a
rapid number of creations/destructions.  The attached patch fixes this.

1) Fix uninitialized next pointer.  This could sometimes cause xenconsoled to
   SEGV on an invalid domain pointer
2) Fix race condition in iterating domain list where removing a domain in a
   callback could lead to the iterators becoming invalid.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>