xen: memory initialization/balloon fixes (#2)

[David Vrabel <david.vrabel@citrix.com>]

This set of patches fixes some bugs in the memory initialization under
Xen and in Xen's memory balloon driver.  They can make 100s of MB of
additional RAM available (depending on the system/configuration).

Patches 1 & 2 are fixes and should be queued for 3.1 and possibly
queued for the 3.0 stable tree.

Patches 3 & 4 increase the amount of low memory in 32 bit domains
started with < 1 GiB of RAM.  Please queue for 3.2

Patch 5 releases all pages in the initial allocation with PFNs that
lie within a 1-1 mapping.  This seems correct to me as I think that
once the 1-1 mapping is set the MFN of the original page is lost so
it's no longer accessible by the kernel (and it cannot be used by
another domain as the Xen still thinks its used by the original
domain).

Changes since #1

- Reordered patches to put "xen: use maximum reservation to limit
  amount of usable RAM" first.
- Check maximum reservation for domU as well.
- New patch "xen/balloon: account for pages released during memory
  setup"
- Added explicit check of size when adding regions in the balloon
  driver.
- New patch "xen: release all pages within 1-1 p2m mappings"

David