Remove some more vestiges of the Xen PV domu support. Specifically,
use vtophys() directly instead of vtomach() and retire the no-longer-used
headers <machine/xenfunc.h> and <machine/xenvar.h>.

Reported by: bde (stale bits in <machine/xenfunc.h>)
Reviewed by: royger (earlier version)
Differential Revision: https://reviews.freebsd.org/D3266