iotests: Add -display none to the qemu options

Without this argument, qemu will print an angry message about not being
able to connect to a display server if $DISPLAY is not set.  For me,
that breaks iotests.supported_formats() because it thus only sees
["Could", "not", "connect"] as the supported formats.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190819201851.24418-2-mreitz@redhat.com
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>