cpus: call the core nmi injection function

We can call the common function here directly since
x86 specific actions will be taken care of by the arch
specific nmi handler

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-Id: <1463761717-26558-4-git-send-email-bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
