[HVM] Move shadow initialisation into domain-creation hypercall.

[HVM] Move shadow initialisation into domain-creation hypercall.
Allocate HVM guest memory in the libxc builder function rather
than in xend. Clean up fall out from these changes.

Todo: Fix ia64. Move PV builder to same model (it should allocate the
memory rather than xend doing so -- then it can avoid using
xc_get_pfn_list()).

Signed-off-by: Keir Fraser <keir@xensource.com>