xen: Remove domain_crash_synchronous() completely

xen: Remove domain_crash_synchronous() completely

domain_crash_synchronous() is unsafe to use in general as it may leave
spinlocks held, temporary memory allocated, etc.

With domain_crash_synchronous() removed from the ARM code in 4.11, take the
opportunity to remove the infrastructure completely by opencoding the softirq
loop in the remaining callsites, all of which are destined for deletion.

None of these sites are at risk of having a pending ioreq to qemu, which means
that the vcpu_end_shutdown_deferral() isn't necessary.

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