VT-d: tidy domid map handling

VT-d: tidy domid map handling

- Correct struct field type.
- Use unsigned int when that suffices.
- Eliminate a (badly typed) local variable from
  context_set_domain_id().
- Don't use -EFAULT inappropriately.
- Move set_bit() such that it won't be done redundantly.
- Constification.
- Reduce scope of some variables.
- Coding style.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>