Move volume ref counting store to a package.

- Add unit tests to make sure the functionality is correct.
- Add FilterByDriver to allow filtering volumes by driver, for future
  `volume ls` filtering and whatnot.

Signed-off-by: David Calavera <david.calavera@gmail.com>

volume/testutils/testutils.go

@@ -0,0 +1,68 @@
+package volumetestutils
+
+import (
+	"fmt"
+
+	"github.com/docker/docker/volume"
+)
+
+// NoopVolume is a volume that doesn't perform any operation
+type NoopVolume struct{}
+
+// Name is the name of the volume
+func (NoopVolume) Name() string { return "noop" }
+
+// DriverName is the name of the driver
+func (NoopVolume) DriverName() string { return "noop" }
+
+// Path is the filesystem path to the volume
+func (NoopVolume) Path() string { return "noop" }
+
+// Mount mounts the volume in the container
+func (NoopVolume) Mount() (string, error) { return "noop", nil }
+
+// Unmount unmounts the volume from the container
+func (NoopVolume) Unmount() error { return nil }
+
+// FakeVolume is a fake volume with a random name
+type FakeVolume struct {
+	name string
+}
+
+// NewFakeVolume creates a new fake volume for testing
+func NewFakeVolume(name string) volume.Volume {
+	return FakeVolume{name: name}
+}
+
+// Name is the name of the volume
+func (f FakeVolume) Name() string { return f.name }
+
+// DriverName is the name of the driver
+func (FakeVolume) DriverName() string { return "fake" }
+
+// Path is the filesystem path to the volume
+func (FakeVolume) Path() string { return "fake" }
+
+// Mount mounts the volume in the container
+func (FakeVolume) Mount() (string, error) { return "fake", nil }
+
+// Unmount unmounts the volume from the container
+func (FakeVolume) Unmount() error { return nil }
+
+// FakeDriver is a driver that generates fake volumes
+type FakeDriver struct{}
+
+// Name is the name of the driver
+func (FakeDriver) Name() string { return "fake" }
+
+// Create initializes a fake volume.
+// It returns an error if the options include an "error" key with a message
+func (FakeDriver) Create(name string, opts map[string]string) (volume.Volume, error) {
+	if opts != nil && opts["error"] != "" {
+		return nil, fmt.Errorf(opts["error"])
+	}
+	return NewFakeVolume(name), nil
+}
+
+// Remove deletes a volume.
+func (FakeDriver) Remove(v volume.Volume) error { return nil }