[PATCH 0 of 5] Add support for mapping grant references into HVM guests (+ some cleanup)

[<steven.smith@eu.citrix.com>]

Add support for mapping grant references into HVM domains, by modifying
the P2M table.

The first couple of patches just tidy up the current grant table
implementation a bit, fixing a couple of bugs in the process, and
should be fairly uncontroversial.  The fourth patch in the series adds
a new HVM op to Xen which allows a HVM guest to map a remote domain's
grant reference into its P2M table, which can then be mapped by the
pagetables in the usual way.  The final patch adds matching support to
the Linux unmodified drivers tree, allowing the new operation to
actually be used, and also adds a couple of very simple test modules.

This isn't actually terribly useful as it stands, because there are no
realistic consumers of this interface.  I wrote it mostly for the
benefit of our closed-source Windows drivers, but that obviously
doesn't help people on the list very much.  I'm not quite sure what
the best way of handling this is; it's clearly better for us for
this stuff to go into xen-unstable, rather than being maintained as an
out-of-tree patch forever, and it'd be a bit of a waste to force
anyone else who wanted this functionality to reinvent it, but it seems
odd to add an interface which has no publicly available consumers.