tools/libxl: Only continue stream operations if the stream is still in progress

tools/libxl: Only continue stream operations if the stream is still in progress

Part of the callback contract with check_all_finished() is that each
running parallel task shall call it exactly once.

Previously, it was possible for stream_continue() or
write_toolstack_record() to fail and call into check_all_finished().  As
the save helpers callback has fired, it no longer counts as in use,
which causes check_all_finished() to fire the stream callback.  Then,
unwinding the stack back and calling check_all_finished() a second time
results in the same conditions being observed, and the stream callback
being fired a second time.

To avoid this, check_all_finished() is called before any other actions
which continue the stream functionality, and the stream is only
continued if it has not been torn down.  This guarantees not to continue
stream operations if the stream does not owe a callback to
check_all_finished().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>