Simply stdout/stderr write logic

[n0toose]

Writes can take place both over serial and over hypercalls. We could treat both of these cases as "writing to serial" from the perspective of Uhyve, thus simplify the logic from the perspective of the host (instead of implementing write for this case)? The downside of that would be that we'd be technically "lying" about serial being, well, serial (UhyveSerial).

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Given that a write hypercall to fd 1 or 2 can also take place, perhaps we should have write use this logic, otherwise it feels like we are implementing write twice. (?)

Originally posted by @n0toose in #1145 (comment)