[XEN] Shadow mode no longer obtains page type references.

[XEN] Shadow mode no longer obtains page type references.
This allows the shadow destructor hook in free_page_type()
to work properly.

Also, move mark_dirty() back to alloc/free_page_type(). It
doesn't matter that this happens before the type count is
modified -- bitmap is extracted by the tools with the domain
paused, so these non-blocking paths are atomic from p.o.v of
the tools.

Signed-off-by: Keir Fraser <keir@xensource.com>