[XEN] Simplify x86_emulate interface.

[XEN] Simplify x86_emulate interface.

 - No distinction between 'special' and 'normal' memory accesses.
 - No reliance on caller-supplied %cr2 value
 - Memory operations include segment identifier to allow callers
   to support non-zero-based segments

TODO:
 1. HVM emulations should take into account segment base, limit, and
    attributes.
 2. We ought to obey stack-size attribute on PUSH/POP instructions.
    Could extend the mode input field, or could add an extra call-out
    hook, or perhaps we don't care at all...

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