tools/python: Python 3 compatibility

tools/python: Python 3 compatibility

convert-legacy-stream is only used for incomming migration from pre Xen 4.7,
and verify-stream-v2 appears to only be used by me during migration
development - it is little surprise that they missed the main converstion
effort in Xen 4.13.

Fix it all up.

Move open_file_or_fd() into a new util.py to avoid duplication, making it a
more generic wrapper around open() or fdopen().

In libxc.py, drop all long() conversion.  Python 2 will DTRT with int => long
promotion, even on 32bit builds.

In convert-legacy-stream, don't pass empty strings to write_record().  Join on
the empty argl will do the right thing.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>