qapi: Open files with encoding='utf-8'

qapi: Open files with encoding='utf-8'

Python 2 happily reads UTF-8 files in text mode, but Python 3 requires
either UTF-8 locale or an explicit encoding passed to open().  Commit
d4e5ec877ca fixed this by setting the en_US.UTF-8 locale.  Falls apart
when the locale isn't be available.

Matthias Maier and Arfrever Frehtes Taifersar Arahesis proposed to use
binary mode instead, with manual conversion from bytes to str.  Works,
but opening with an explicit encoding is simpler, so do that.

Since Python 2's open() doesn't support the encoding parameter, we
need to suppress it with a version check.

Reported-by: Arfrever Frehtes Taifersar Arahesis <[email protected]>
Reported-by: Matthias Maier <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20180618175958[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: Eric Blake <[email protected]>