xen/domain: Introduce domain_teardown()

xen/domain: Introduce domain_teardown()

There is no common equivelent of domain_reliquish_resources(), which has
caused various pieces of common cleanup to live in inappropriate
places.

Perhaps most obviously, evtchn_destroy() is called for every continuation of
domain_reliquish_resources(), which can easily be thousands of times.

Create domain_teardown() to be a new top level facility, and call it from the
appropriate positions in domain_kill() and domain_create()'s error path.  The
intention is for this to supersede domain_reliquish_resources() in due course.

No change in behaviour yet.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>