noux: Keep track of how dataspaces are used (WiP)

This patch eliminates the "no attachment at..." warnings, which
were caused by a use-after-free problem of dataspaces. When a
dataspace was destroyed, the users of the dataspace were not
informed and therefore could not revert possible attachments to
RM sessions. The fix introduces a callback mechanism that allows
dataspace users (i.e., RM regions) to register for the event that
a dataspace vanishes.

Right now, the patch handles RAM dataspaces and managed RM dataspaces.
The handling of ROM dataspaces is still missing because this requires
the not-yet existing virtualization of the ROM service.

Issue #485