[HVM] add super page support for HVM migration

[HVM] add super page support for HVM migration

This patch try to allocate 2M pages on target side based on analysis
of pfn sequence sent from source side for HVM migration.

The algorithm is: If pseudo-phys page is not yet populated in target
domain, AND it is first page of a 2MB extent, AND no other pages in
that extent are yet populated, AND the next pages in the save-image
stream populate that extent in order, THEN allocate a super page. If
the next 511 pages (to make the 2MB extent) are split across a batch
boundary, we have to optimistically allocate a super page in this
batch, and then break it into several 4K pages in the next batch,
which is speculative.

This patch is also friendly to PV guest migration.

Signed-Off-By: Zhai Edwin <edwin.zhai@intel.com>