xen/domain: Break _domain_destroy() out of domain_create() and complete_domain_destroy()

xen/domain: Break _domain_destroy() out of domain_create() and complete_domain_destroy()

This is the first step in making the destroy path idempotent, and using it in
place of the ad-hoc cleanup paths in the create path.

To begin with, the trivial free operations are broken out.  The rest of the
cleanup code will be moved as it is demonstrated (or made) to be idempotent.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>