x86/irq: skip unmap_domain_pirq XSM during destruction

x86/irq: skip unmap_domain_pirq XSM during destruction

xsm_unmap_domain_irq was seen denying unmap_domain_pirq when called from
complete_domain_destroy as an RCU callback.  The source context was an
unexpected, random domain.  Since this is a xen-internal operation,
going through the XSM hook is inapproriate.

Check d->is_dying and skip the XSM hook when set since this is a cleanup
operation for a domain being destroyed.

Suggested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>