[XEN][POWERPC] allocate HTAB using shadow calls

[XEN][POWERPC] allocate HTAB using shadow calls

This patch will use the shadow_ops call to allocate the domains Hashed
Page-table.  This allows for the management tools to customize its size
for the domain.

Warning! The management tools have yet to be updated to use this
properly and update the devtree accordingly so code will assume 64M
RAM and 1M HTAB while the shadow up continues to receive a 0 request.

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>