tests/qemu-iotests: Update tests to recent desugarized -accel option

tests/qemu-iotests: Update tests to recent desugarized -accel option

Commit 6f6e1698a6 desugarized "-machine accel=" to a list
of "-accel" options. Since now "-machine accel" and "-accel"
became incompatible, update the iotests to the new format.

Error reported here:
https://gitlab.com/qemu-project/qemu/-/jobs/385801004#L3400

Reported-by: GitLab CI
Fixes: 6f6e1698a6 (vl: configure accelerators from -accel options)
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200106130951.29873-1-philmd@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>