Add event logging to libpod, even display to podman

In lipod, we now log major events that occurr.  These events
can be displayed using the `podman events` command. Each
event contains:

* Type (container, image, volume, pod...)
* Status (create, rm, stop, kill, ....)
* Timestamp in RFC3339Nano format
* Name (if applicable)
* Image (if applicable)

The format of the event and the varlink endpoint are to not
be considered stable until cockpit has done its enablement.

Signed-off-by: baude <bbaude@redhat.com>

libpod/image/prune.go

@@ -1,6 +1,9 @@
 package image
 
-import "github.com/pkg/errors"
+import (
+	"github.com/containers/libpod/libpod/events"
+	"github.com/pkg/errors"
+)
 
 // GetPruneImages returns a slice of images that have no names/unused
 func (ir *Runtime) GetPruneImages(all bool) ([]*Image, error) {
@@ -41,6 +44,7 @@ func (ir *Runtime) PruneImages(all bool) ([]string, error) {
 		if err := p.Remove(true); err != nil {
 			return nil, errors.Wrap(err, "failed to prune image")
 		}
+		defer p.newImageEvent(events.Prune)
 		prunedCids = append(prunedCids, p.ID())
 	}
 	return prunedCids, nil