x86: re-connect VCPUOP_send_nmi for 32-bit guests

x86: re-connect VCPUOP_send_nmi for 32-bit guests

With the "inversion" of VCPUOP handling, processing arch-specific ones
first, the forwarding of this sub-op from the (common) compat handler to
(common) non-compat one did no longer have the intended effect. It now
needs forwarding between the arch-specific handlers.

Fixes: 8a96c0ea7999 ("xen: move do_vcpu_op() to arch specific code")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>