x86: restore (optional) forwarding of PCI SERR induced NMI to Dom0

x86: restore (optional) forwarding of PCI SERR induced NMI to Dom0

c/s 22949:54fe1011f86b removed the forwarding of NMIs to Dom0 when they
were caused by PCI SERR. NMI buttons as well as BMCs (like HP's iLO)
may however want such events to be seen in Dom0 (e.g. to trigger a
dump).

Therefore restore most of the functionality which named c/s removed
(adjusted for subsequent changes, and adjusting the public interface to
use the modern term, retaining the old one for backwards
compatibility).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Keir Fraser <keir@xen.org>