xen/grant-table: xen_shim_domain() support

xen/grant-table: xen_shim_domain() support

With xen-shim, allocate_xenballooned_pages() only allocates a
place-holder page (pfn 0) expecting a subsequent map_grant_ref to fix
it up.

However, this means that, until the grant operation
(GNTTABOP_map_grant_ref) provides a valid page, we cannot set
PagePrivate or save any state.

This patch elides the setting of that state if xen_shim_domain(). In
addition, gnttab_map_refs() now fills in the appropriate page returned
from the grant operation.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>