libosmo-simtrace2 traditionally had only supported blocking, synchronous
I/O, while other osmocom programs such as remsim-client used
asynchronous USB I/O.
Using async USB I/O for IRQ + IN transfers while using blocking I/O for
OUT transfers doesn't seem to work reliably, so we have to offer a way
to perform the OUT transfers generated within libosmo-simtrace2 in async