hvt_module_net: do not return EAGAIN on a read of 0 bytes

[hannesm]

fixes #559 //cc @reynir

[dinosaure]

I'm really not sure about that, we should really look into the expected behavior between reading on a TAP interface and what the unikernel expect. I would really like to emphasise that #559 points a behaviour that is not expected if a file descriptor is used instead of a tap interface (which is also not expected).

[hannesm]

expected behavior between reading on a TAP interface and what the unikernel expect

I did, so mirage-net-solo5 behaves fine with an empty buffer received.

I also tried to lookup when a read of a tap interface will return 0, without success (so I couldn't find any documentation or code).

For other bindings, I can't see any special path if the network read returns 0.

[hannesm]

But I'm also fine to close this, and the issue 559.