Implement nmi function in monitor

both HMP and QMP
Changlog:
use self.cmd() on QMP monitor

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
Acked-by: Lukas Doktor <ldoktor@redhat.com>
Update kdump case to use vm.monitor.nmi()

This would prevent case error when test with QMP
as it uses 'inject-nmi' instead of 'nmi'

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
Acked-by: Lukas Doktor <ldoktor@redhat.com>