This **CHANGES** the human monitor "nmi" command behavior.

Currently it accepts an CPU argument which, when provided, will send
the NMI to the specified CPU. This feature is of discussable value
though and HMP shouldn't have more features than QMP, so let's use
QMP's instead (it's also simpler).

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>