xl: ensure handle_domain & preserve_domain update the global domid not a shadow

xl: ensure handle_domain & preserve_domain update the global domid not a shadow

xl keeps the current domid in a global variable, however it also has various
functions (including the two above) which take a domid parameter which shadows
this.

This fixes the issue introduced by 25563:dbf54d93ac40 "xl: initialise domid to
an explicitly invalid value" but does not tackle the wider problem.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>