Implement --device idType://id for ctr run on Windows

Also fixes the issue that `ctr run` on Windows offered help for the
non-Windows implementation, but was silently ignored.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

cmd/ctr/commands/commands_windows.go

@@ -24,5 +24,8 @@ func init() {
 	ContainerFlags = append(ContainerFlags, cli.Uint64Flag{
 		Name:  "cpu-count",
 		Usage: "number of CPUs available to the container",
+	}, cli.StringSliceFlag{
+		Name:  "device",
+		Usage: "identifier of a device to add to the container  (e.g. class://5B45201D-F2F2-4F3B-85BB-30FF1F953599)",
 	})
 }