I was looking into fixing all the pmaps to pay attention to the
flags args to pmap_enter() (PR 25640), and I noticed that the xen/i386
pmap has a pmap_enter_ma() function that has the same problem but
is apparently unused.  is there any reason to keep this around?

I also noticed that there are various other remnants of the separate
xen release (eg. distrib/xen, etc/etc.xen).  now that xen is part of
the i386 release, is there any reason to keep these other bits around?

it seems to me that it also might be better to move the i386 xen bits
from arch/xen/i386 to arch/i386/xen, and move the xen-specific device
drivers from arch/xen/xen to dev/xen or somesuch, but I'll leave that
for someone else.

-Chuck