virtio-console code marked as `Safe` + `Sync` but code is not thread-safe

[andrisaar]

MmioConsoleChannel implements unsafe traits Send and Sync directly. For now, this is safe as we don't have threads in the kernel, but in the future that may not be the case.

This bug is to remind us of the potential foot-gun lurking in the code and breadcrumbs to fix it when we need it.

[tiziano88]

@andrisaar @conradgrobler is this still a potential issue?

[conradgrobler]

I think it is. At the moment we don't have plans to add multiple threads, but we need to revisit this when we do.