xen/domain: Introduce vcpu_teardown()

xen/domain: Introduce vcpu_teardown()

Similarly to c/s 98d4d6d8a6 "xen/domain: Introduce domain_teardown()",
introduce a common mechanism for restartable per-vcpu teardown logic.

Extend the PROGRESS() mechanism to support saving and restoring the vcpu loop
variable across hypercalls.

This will eventually supersede domain_reliquish_resources(), and reduce the
quantity of redundant logic performed.

No functional change (yet).

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