Remove the PHYSDEV_ACCESS configure option from Linux.

Remove the PHYSDEV_ACCESS configure option from Linux.
It saves very little space, no distro will disable it
in their unified build, and it's rather a pain to
implement it properly.

I've left the PRIVILEGED_GUEST option for now, as it
provides some indication of where our platform-bringup
hacks are in our Linux tree. We may wish to exclude
those from upstream merge in the first instance.

Signed-off-by: Keir Fraser <keir@xensource.com>