This is a continuation of review in #6206 (comment), and is the prerequisite of moby/moby#50942.

Instead of exposing an overly broad SpecOpts, this PR only exposes an Isolated flag to make buildkit create containers in Hyper-V VM in Windows.

The name Isolated is taken from the flag --isolated from ctr command in containerd:

var platformRunFlags = []cli.Flag{
	&cli.BoolFlag{
		Name:  "isolated",
		Usage: "Run the container with vm isolation",
	},
}

if cliContext.Bool("isolated") {
	opts = append(opts, oci.WithWindowsHyperV)
}