x86: Allow bitop functions to be applied only to fields of at least 4

x86: Allow bitop functions to be applied only to fields of at least 4
bytes. Otherwise the 'longword' processor instructions used will
overlap with adjacent fields with unpredictable consequences.

This change requires some code fixup and just a few casts (mainly when
operating on guest-shared fields which cannot be changed, and which by
observation are clearly safe).

Based on ideas from Jan Beulich <jbeulich@novell.com>

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