vmx: fix handling of NMI VMEXIT.

vmx: fix handling of NMI VMEXIT.

Call do_nmi() directly and explicitly re-enable NMIs rather than
raising an NMI through the APIC. Since NMIs are disabled after the
VMEXIT, the raised NMI would be blocked until the next IRET
instruction (i.e. the next real interrupt, or after scheduling a PV
guest) and in the meantime the guest will spin taking NMI VMEXITS.

Also, handle NMIs before re-enabling interrupts, since if we handle an
interrupt (and therefore IRET) before calling do_nmi(), we may end up
running the NMI handler with NMIs enabled.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>