Append osv::clock::uptime timestamps to debug messages

[tgrabiec]

In the cloud we don't see "live" output, we can only get the whole dmesg buffer, so it's hard to asses timing of messages. I think it would be useful to have timestamps before each line in the output of /os/dmesg.

[dhruvsethi227]

Hello, my name is Dhruv and I am a senior computer science student at UT Austin. My group and I were wondering if we could tackle this issue as part of our virtualization class assignment.

[wkozaczuk]

Go for it.