tools/libxl: Infrastructure to convert a legacy stream

tools/libxl: Infrastructure to convert a legacy stream

Provide a thin wrapper around exec()ing the python conversion utility,
and a stub implementation for cases where conversion is not wanted
(i.e. not x86).

One complication is that the caller of this interface needs to assume
ownership of the output fd, to prevent it being closed while still in
use in a datacopier.

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: Formatting tweaks, description of the width field.
v3: Trimmed down massively from v2, provide libxl_no_convert_callout.c