ukvm/console: return actual number of bytes written

[hannesm]

instead of the string length

[hannesm]

this addresses mirage/mirage-flow#4 (comment) and comes along with mirage/mirage-solo5#19 and mirage/mirage-console-solo5#12 (but this change in here is very independent, and does not change any API)

[mato]

There are multiple issues with the solo5.h APIs, I'm working on a minimal refresh that corrects the most egregious problems without actually redesigning anything.

In the specific case of the console, I'm going to change solo5_console_write() to just return void; I don't see any point in trying to "recover" from failed console writes.

PR coming up once it's ready, we can discuss the changes there.

[hannesm]

my motivation was to unify the semantics of console writes across backends, please see mirage/mirage-flow#4 (comment) for more information. but i agree that error recovery from partial writes is slightly overengineering -- but I'd still like to have a unified semantics here (as in: writing to the console may fail or be partial), and properly documented error cases.

[hannesm]

superseeded by #201