qmp-event: add event notification for memory hot unplug error

When memory hot unplug fails, this patch adds support to send
QMP event to notify mgmt about this failure.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>