virtcontainers: change container's rootfs from string to mount alike struct

container's rootfs is a string type, which cannot represent a
block storage backed rootfs which hasn't been mounted.
Change it to a mount alike struct as below:
    RootFs struct {
            // Source specify the BlockDevice path
            Source string
            // Target specify where the rootfs is mounted if it has been mounted
            Target string
            // Type specifies the type of filesystem to mount.
            Type string
            // Options specifies zero or more fstab style mount options.
            Options []string
            // Mounted specifies whether the rootfs has be mounted or not
            Mounted bool
     }

If the container's rootfs has been mounted as before, then this struct can be
initialized as: RootFs{Target: <rootfs>, Mounted: true} to be compatible with
previous case.

Fixes:#1158

Signed-off-by: lifupan <lifupan@gmail.com>

virtcontainers/pkg/oci/utils.go

@@ -510,17 +510,17 @@ func SandboxConfig(ocispec CompatOCISpec, runtime RuntimeConfig, bundlePath, cid
 // ContainerConfig converts an OCI compatible runtime configuration
 // file to a virtcontainers container configuration structure.
 func ContainerConfig(ocispec CompatOCISpec, bundlePath, cid, console string, detach bool) (vc.ContainerConfig, error) {
-
 	ociSpecJSON, err := json.Marshal(ocispec)
 	if err != nil {
 		return vc.ContainerConfig{}, err
 	}
 
-	rootfs := ocispec.Root.Path
-	if !filepath.IsAbs(rootfs) {
-		rootfs = filepath.Join(bundlePath, ocispec.Root.Path)
+	rootfs := vc.RootFs{Target: ocispec.Root.Path, Mounted: true}
+	if !filepath.IsAbs(rootfs.Target) {
+		rootfs.Target = filepath.Join(bundlePath, ocispec.Root.Path)
 	}
-	ociLog.Debugf("container rootfs: %s", rootfs)
+
+	ociLog.Debugf("container rootfs: %s", rootfs.Target)
 
 	cmd := types.Cmd{
 		Args:            ocispec.Process.Args,

virtcontainers/pkg/oci/utils_test.go

@@ -200,7 +200,7 @@ func TestMinimalSandboxConfig(t *testing.T) {
 
 	expectedContainerConfig := vc.ContainerConfig{
 		ID:             containerID,
-		RootFs:         path.Join(tempBundlePath, "rootfs"),
+		RootFs:         vc.RootFs{Target: path.Join(tempBundlePath, "rootfs"), Mounted: true},
 		ReadonlyRootfs: true,
 		Cmd:            expectedCmd,
 		Annotations: map[string]string{