PoD memory 9/9: xend integration

PoD memory 9/9: xend integration

Xend integration for PoD functionality.
* Add python bindings for xc_hvm_domain_build() and
xc_domain_memory_set_pod_target()
* Always call xc_hvm_domain_build(), with memsize = memory_static_max
and target=memory_dynamic_max
* When setting a new memory target:
 + First make sure we actually have enough free memory for the target
setting to succeed
 + Call set_pod_target() with the new target, to Xen can do the Right
 Thing.

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