xen/gntdev: add VM_PFNMAP to vma

These pages are from other domains, so don't have any local PFN.
VM_PFNMAP is the closest concept Linux has to this.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>