use 'Hardware domain' instead of 'Domain 0' in hwdom_shutdown()

use 'Hardware domain' instead of 'Domain 0' in hwdom_shutdown()

hwdom_shutdown() operates with hardware domains, use the proper wording.
Eliminate pointless braces from switch cases.

Use hardware_domain->domain_id instead of hardware_domid to print the actual
domain ID as in some cases it can differ (e.g. Dom0 dies before the actual HW
domain got created, kexec for the HW domain is being performed,...).

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>