x86/nmi: fix shootdown of pcpus running in VMX non-root mode

x86/nmi: fix shootdown of pcpus running in VMX non-root mode

c/s 7dd3b06ff "vmx: fix handling of NMI VMEXIT" fixed one issue but
inadvertently introduced a regression when it came to the NMI shootdown.  The
shootdown code worked by patching vector 2 in each IDT, but the introduced
direct call to do_nmi() bypassed this.

Instead of patching each IDT, take a different approach by updating the
existing dispatch table.  This allows for the removal of the remote IDT
patching and the removal of the nmi_crash() entry point.

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