nmi: remove x86 specific nmi handling

nmi_monitor_handle is wired to call the x86 nmi
handler. So, we can directly use it at call sites.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-Id: <1463761717-26558-3-git-send-email-bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>