Add (or extend existing) API to read/write guest's xenstore directory (realtime)

[TSnake41]

While add_to_xenstore_data/get_xenstore_data/set_xenstore_data can add new entries to xenstore, they are only applied on guest start.
It can be useful to have a similar (or extend existing) interface to interact with xenstore entries while the guest is alive (eventually triggering WATCH_EVENT in the guest) without having to rely on manual xenstore-read/write commands that needs to be run with a proper path and on the proper host.