kvm: Enable user space NMI injection for kvm guest

Make use of the new KVM_NMI IOCTL to send NMIs into the KVM guest if the
user space raised them. (example: qemu monitor's "nmi" command)

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
