xen/x86: add nmi continuation framework

xen/x86: add nmi continuation framework

Actions in NMI context are rather limited as e.g. locking is rather
fragile.

Add a framework to continue processing in normal interrupt context
after leaving NMI processing.

This is done by a high priority interrupt vector triggered via a
self IPI from NMI context, which will then call the continuation
function specified during NMI handling.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>