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 <firstname.lastname@example.org>, 2014-01-14 06:44:08
Approach proposed by: Roger Pau Monné <royger@FreeBSD.org>