qapi: convert to use python print function instead of statement

Python 3 no longer supports the bare "print" statement, it must be
called as a normal function with round brackets. It is possible to
opt-in to this new syntax with Python 2.6 onwards by importing the
"print_function" from the "__future__" module, making it easy to
support Python 2 and 3 in parallel.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180116134217.8725-2-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>