rename dom0 to hardware_domain

rename dom0 to hardware_domain

This should not change any functionality other than renaming the global
variable.  In a few cases (primarily the domain building code), a local
variable or argument named dom0 was created and used instead of the
global hardware_domain to clarify that the domain being used in this
case is actually domain 0.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Christoph Egger <chegger@amazon.de>
Acked-by: Keir Fraser <keir@xen.org>