Remove dependency on tar2vhd for LCOW differ

This change no longer requires the use of a UtlityVM on Windows to convert the
layer tar to an ext4 vhd for LCOW. This has a significant performance boost
that makes linux/amd64 layer extraction comparable to native Linux performance.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>

vendor/github.com/Microsoft/hcsshim/internal/runhcs/util.go

@@ -0,0 +1,16 @@
+package runhcs
+
+import "net/url"
+
+const (
+	SafePipePrefix = `\\.\pipe\ProtectedPrefix\Administrators\`
+)
+
+// ShimSuccess is the byte stream returned on a successful operation.
+var ShimSuccess = []byte{0, 'O', 'K', 0}
+
+func SafePipePath(name string) string {
+	// Use a pipe in the Administrators protected prefixed to prevent malicious
+	// squatting.
+	return SafePipePrefix + url.PathEscape(name)
+}