tools/libxl: Convert a legacy stream if needed

tools/libxl: Convert a legacy stream if needed

For backwards compatibility, a legacy stream needs converting before
it can be read by the v2 stream logic.

This causes the v2 stream logic to need to juggle two parallel tasks.
check_all_finished() is introduced for the purpose of joining the
tasks in both success and error cases.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
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v4: Alter logic layout in check_all_finished()
v3: Simplified greatly from v2.  No practical change.