xen: create VM_MEMATTR_XEN for Xen memory mappings

The requirements for pages shared with Xen/other VMs may vary from
architecture to architecture. As such create a macro which various
architectures can use.

Remove a use of PAT_WRITE_BACK in xenstore.c. This is a x86-ism which
shouldn't have been present in a common area.

Original idea: Julien Grall <julien@xen.org>, 2014-01-14 06:44:08
Approach suggested by: royger
Reviewed by: royger, mhorne
Differential Revision: https://reviews.freebsd.org/D29351