Replaying virtual nodes bootup

[eduble]

Since commit d792215 serial output of nodes is recorded in /var/lib/walt/logs/vnodes, thus they can be replayed with scriptreplay.
It would be better to reuse the logging system for that, because /var/lib/walt/logs/vnodes is only accessible on walt server machine, and using them is not user-friendly.
Instead we could:

• let the server accept incoming log lines from a local unix socket (using it would avoid loading the network)
• let walt-virtual-node send log lines there, with logstream serial, when virtual node is run on the server
• add a command option --replay to walt log show. When used, walt client would follow delays between the first log line and the others. Using this option on logstream serial would allow to "replay" the bootup of a node.