qemu.py: Fix syntax error

Python requires parentheses around multiline expression. This fixes the
breakage of all Python-based qemu-iotests cases that was introduced in
commit dab91d9a.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20170918052524.4045-1-kwolf@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>