[PATCH 0 of 2 RFC] Rework populate-on-demand sweeping

[George Dunlap <george.dunlap@eu.citrix.com>]

Rework populate-on-demand sweeping

Last summer I did some work on testing whether our PoD sweeping code
was achieving its goals: namely, never crashing unnecessairly,
minimizing boot time, and maximizing the number of superpages in the
p2m table.

This is one of the resulting patch series.  

I'm posting it to make sure that maintainers think it's still suitable
for inclusion in 4.2.  The patces against 4.1 have been extensively in
the XenServer testing framework and have been in use by XenServer
customers for over 9 months now.  But the p2m code has changed
extensively in that time, so one could argue that the testing doesn't
give us the same degree of confidence in the patches against 4.2 as
against 4.1.  (On the other hand, the PoD code hasn't changed that
much.)

I haven't done more than compile-test it at this point, so please just
review ideas and "is this 4.2 material".  If I get positive feedback,
I'll do more testing and re-submit.