vm: always capture last executed programs

[dvyukov]

trafficstars

Currently we merge syz-fuzzer output and kernel console output into one stream. If kernel produces way too much output before a crash (e.g. a stream of OOM messages), all programs are evicted from the log. It would be useful if we always capture at least some amount of the last executed programs. To do this we probably need to return 2 separate output channels from vm.Run and then do this more complex merging in vm.MonitorExecution. This will require quite radical changes in vm. Can make sense to consider this with #607 and simplify/unify some things along the way.

[dvyukov]

Another example we go:

[  118.978828][ T3600] bridge0: port 2(bridge_slave_1) entered disabled state
[  118.987933][ T3600] bridge0: port 1(bridge_slave_0) entered disabled state
[  119.018985][ T3610] ======================================================
[  119.018985][ T3610] WARNING: the manclient_loop: send disconnect: Broken pipe

"client_loop" is qemu/ssh output (?) Related to #2377 @a-nogikh