[XEN] Don't keep shadows of PAE guest l3 tables.

[XEN] Don't keep shadows of PAE guest l3 tables.

Holding pages readonly that have guest PAE l3 tables in them means
a performance hit and potential bug if the guest puts other
datastructures on the same page as an l3 table.  Instead of shadowing
them, treat PAE guests as if they had four CR3 registers, and load all
four top-level entries when we handle a CR3 write.  This also cuts about
500 lines of special-case refcounting and re-copying code.

Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>