Avocado not killing QEMU processes on test timeouts

[Peter Maydell]

I was testing 'make check-avocado' on an s390 host, which currently
has a bunch of tests that time out and fail INTERRUPTED because of
various endianness bugs. I found that when the avocado run had
finished, it hadn't cleaned up the various qemu-system-foo
processes that the tests spawned (and so they kept on running
using CPU in the background).

What's the mechanism that's supposed to clean this up? I think
that the top level avocado process should never exit without
making sure all the processes that got spawned have been
killed (with SIGKILL if necessary).

thanks
-- PMM