Make map_domain_page_global fail

Make map_domain_page_global fail

When the global mapping cache runs out, make map_domain_page_global
return NULL on failure rather than fire an assertion failure.  This
also updates the callers to handle the error gracefully.

The only exception to this is the shadow pagetable code, which uses
map_domain_page_global to create a mapping for
v->arch.paging.shadow.guest_vtable; it's not clear this needs to be a
global mapping anyway.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>