So drivers can clear their mem io table entries on exit back to unassigned
state.

Also make the io mem index allocation dynamic.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6601 c046a42c-6fe2-441c-8c8c-71466251a162