WIP xendevicemodel.h

WIP xendevicemodel.h

This is just the header docs as a starting point for discussion for
some further interfaces.

Implementation notes:

xendevicemodel_create_vram_region:
   - xen-hvm.c:xen_add_to_physmap, only used for vram (explicit check).
        - XENMEM_add_to_physmap + XEN_DOMCTL_pin_mem_cacheattr
        - Is first XENMEM_populate_physmap'd in
          xen-hvm.c:xen_ram_alloc. Some refactoring required.
   - XEN_DOMCTL_pin_mem_cacheattr needs to become a stable API
   - Using XEN_DOMCTL_MEM_CACHEATTR_WB.
   - xc_domain_pin_memory_cacheattr can likely be removed from libxc

xendevicemodel_destroy_vram_region:
   - xen-hvm.c:xen_remove_from_physmap, implicit check via presence of
     physmap, which is only for vram (see xen_add_to_physmap)
        - XENMEM_add_to_physmap
        - Don't need to undo pin_mem_cacheattr?