x86/emul: Simplfy emulation state setup

x86/emul: Simplfy emulation state setup

The current code to set up emulation state is ad-hoc and error prone.

 * Consistently zero all emulation state structures.
 * Avoid explicitly initialising some state to 0.
 * Explicitly identify all input and output state in x86_emulate_ctxt.  This
   involves rearanging some fields.
 * Have x86_decode() explicitly initalise all output state at its start.

While making the above changes, two minor tweaks:

 * Move the calculation of hvmemul_ctxt->ctxt.swint_emulate from
   _hvm_emulate_one() to hvm_emulate_init_once().  It doesn't need
   recalculating for each instruction.
 * Change force_writeback to being a boolean, to match its use.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>