HVM can set some xenstore entries (in qubes-tools/ subtree) to pass
informations about installed tools to dom0. qubes.NotifyTools service
triggers update of VM properties (like qrexec_installed).
This way, after installation of Qubes Windows Tools, the user doesn't need
to change any VM settings to use the tools.
If not marked as installed, start it in background - waiting for
possible Qubes Tools installation. If that happens, make it possible to
tell the dom0 about that fact.
