XEN/amd64 may initiate i/o over the pages not mapped by the direct
map. Handle busdma bouncing and ata PIO accesses by using global
frame used by the current CPU locally for the duration of
pmap_quick_enter/remove_page(). A spin mutex protects the concurent
frame use and prevents thread migration.

Noted by: royger
Reviewed by: alc, jah, royger (previous version)
Sponsored by: The FreeBSD Foundation