tools: Introduce "allocate-only" page type for migration

tools: Introduce "allocate-only" page type for migration

To detect presence of superpages on the receiver side, we need
to have strings of sequential pfns sent across on the first iteration
through the memory.  However, as we go through the memory, more and
more of it will be marked dirty, making it wasteful to send those pages.

This patch introduces a new PFINFO type, "XALLOC".  Like PFINFO_XTAB, it
indicates that there is no corresponding page present in the subsquent
page buffer.  However, unlike PFINFO_XTAB, it contains a pfn which should be
allocated.

This new type is only used for migration; but it's placed in
xen/public/domctl.h so that the value isn't reused.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>