PoD memory 7/9: Xen interface

PoD memory 7/9: Xen interface

Implement Xen interface to PoD functionality.
* Increase the number of MEMOP bits from 4 to 6 (increasing the number
of available memory operations from 16 to 64).
* Introduce XENMEMF_populate_on_demand, which will cause
populate_physmap() to fill a range with PoD entries rather than
backing it with ram
* Introduce XENMEM_[sg]et_pod_target operation to the memory
hypercall, to get and set PoD cache size.  set_pod_target() should be
called during domain creation, as well as after modifying the memory
target of any domain which may have outstanding PoD entries.

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