The virDomainObjPtr object stores state about a running domain.
This object is shared across all drivers so it is not appropriate
to include driver specific state here. This patch adds the ability
to request a blob of private data per domain object instance. The
driver must provide a allocator & deallocator for this purpose

THis patch abuses the virCapabilitiesPtr structure for storing the
allocator/deallocator callbacks, since it is already being abused
for other internal things relating to parsing. This should be moved
out into a separate object at some point.

* src/conf/capabilities.h: Add privateDataAllocFunc and
  privateDataFreeFunc fields
* src/conf/domain_conf.c: Invoke the driver allocators / deallocators
  when creating/freeing virDomainObjPtr instances.
* src/conf/domain_conf.h: Pass virCapsPtr into virDomainAssignDef
  to allow access to the driver specific allocator function
* src/lxc/lxc_driver.c, src/opennebula/one_driver.c,
  src/openvz/openvz_driver.c, src/qemu/qemu_driver.c,
  src/test/test_driver.c, src/uml/uml_driver.c: Update for
  change in virDomainAssignDef contract