PoD memory 8/9: libxc interface

PoD memory 8/9: libxc interface

Implement libxc interface to PoD functionality:
* Add xc_hvm_build_target_mem(), which takes both memsize and target.
Memsize is the total memory, allocated in PoD pages and reported in
the e820; target is the size of the cache.  If these are the same, the
normal funcitonality is called.  (So you can use the same function to
build always, and it will decide whether to use PoD or not.)
* Add xc_domain_memory_[gs]et_pod_target(), which sets and/or returns
information about the PoD cache and p2m entries.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>