add migration state change notifiers

add migration state change notifiers

This patch adds functions to register and unregister notifiers for
migration state changes and a function to query the migration state.
The notifier is called on every state change.  Once after establishing a
new migration object (which is in active state then) and once when the
state changes from active to completed, canceled or error.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>