Windows: Fixes Windows containers with image volumes

Currently, there are few issues that preventing containers
with image volumes to properly start on Windows.

- Unlike the Linux implementation, the Container volume mount paths
  were not created if they didn't exist. Those paths are now created.

- while copying the image volume contents to the container volume,
  the layers were not properly deactivated, which means that the
  container can't start since those layers are still open. The layers
  are now properly deactivated, allowing the container to start.

- even if the above issue didn't exist, the Windows implementation of
  mount/Mount.Mount deactivates the layers, which wouldn't allow us
  to copy files from them. The layers are now deactivated after we've
  copied the necessary files from them.

- the target argument of the Windows implementation of mount/Mount.Mount
  was unused, which means that folder was always empty. We're now
  symlinking the Layer Mount Path into the target folder.

- hcsshim needs its Container Mount Paths to be properly formated, to be
  prefixed by C:. This was an issue for Volumes defined with Linux-like
  paths (e.g.: /test_dir). filepath.Abs solves this issue.

Signed-off-by: Claudiu Belu <cbelu@cloudbasesolutions.com>

mount/mount_windows.go

@@ -18,6 +18,7 @@ package mount
 
 import (
 	"encoding/json"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -50,15 +51,21 @@ func (m *Mount) Mount(target string) error {
 	if err = hcsshim.ActivateLayer(di, layerID); err != nil {
 		return errors.Wrapf(err, "failed to activate layer %s", m.Source)
 	}
-	defer func() {
-		if err != nil {
-			hcsshim.DeactivateLayer(di, layerID)
-		}
-	}()
 
 	if err = hcsshim.PrepareLayer(di, layerID, parentLayerPaths); err != nil {
 		return errors.Wrapf(err, "failed to prepare layer %s", m.Source)
 	}
+
+	// We can link the layer mount path to the given target. It is an UNC path, and it needs
+	// a trailing backslash.
+	mountPath, err := hcsshim.GetLayerMountPath(di, layerID)
+	if err != nil {
+		return errors.Wrapf(err, "failed to get layer mount path for %s", m.Source)
+	}
+	mountPath = mountPath + `\`
+	if err = os.Symlink(mountPath, target); err != nil {
+		return errors.Wrapf(err, "failed to link mount to taget %s", target)
+	}
 	return nil
 }