libxl: allow libxl to calculate shadow mem requirements
Long, long ago before libxl_get_required_shadow_memory() was
made publicly available, its code was copied to the libxl driver
for calculating shadow memory requirements of HVM domains.
Long ago, libxl_get_required_shadow_memory() was exported in
libxl_utils.h and included in xen-devel packages everywhere.
Remove the copied code, which has become stale, and let libxl
provode a proper shadow memory value.