miscellaneous cleanup

miscellaneous cleanup

... noticed while putting together the 16Tb support patches for x86.

Briefly, this (in order of the changes below)
- fixes an inefficiency in x86's context switch code (translations to/
  from struct page are more involved than to/from MFNs)
- drop unnecessary MFM-to-page conversions
- drop a redundant call to destroy_xen_mappings() (an indentical call
  is being made a few lines up)
- simplify a VA-to-MFN translation
- drop dead code (several occurrences)
- add a missing __init annotation

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>