x86/xen: fallback when VCPUOP_send_nmi is not available

It has been reported that on some AWS instances VCPUOP_send_nmi
returns -38 (ENOSYS). The hypercall is only available for HVM guests
in Xen 4.7 and newer. Add a fallback to use the native NMI sending
procedure when VCPUOP_send_nmi is not available, so that the NMI is
not lost.

Reported and Tested by: avg
MFC after: 1 week
Fixes: b2802351c162 ('xen: fix dispatching of NMIs')
Sponsored by: Citrix Systems R&D