xen: use correct cache attributes for Xen specific memory regions

bus_activate_resource maps memory regions as uncacheable on x86, which
is more strict than required for regions allocated using xenmem_alloc,
so don't rely on bus_activate_resource and instead map the region
using pmap_mapdev_attr and VM_MEMATTR_XEN as the cache attribute.

Sponsored by: Citrix Systems R&D