In the shadow mode, thanks to recent patches, the shadow

In the shadow mode, thanks to recent patches, the shadow
reference counts (both general counts and type counts)
are now correct in all of my testing so far.  As a result,
we should remove the shadow_tainted_refcnt flag from
the domain structure.  Leaving this flag in place would
create confusion for programmers and would also prevent
future debugging of problems in handling general and type
reference counts in the shadow mode.  In addition, the
patch also (1) drops the requirement that shadow_mode_disable()
should only be called in the domain destruction sequence
(even though it is only used that way so far), and
(2) strictly assert that the type reference count must
be 0 when we free a page from the dom heap.

Signed-off-by: Khoa Huynh <khoa@us.ibm.com>