recent flakiness (intermittent hangs) of migration-test

[Peter Maydell]

Recently I've seen the 'make check' migration-test developing an
intermittent hang; I've seen this now on aarch32 host several times
and also on s390x host. The symptom is that the test just hangs
with a couple of child qemu processes sitting around doing nothing.
^Cing out of 'make check' doesn't kill the qemu processes; they
seem to need a 'kill -9'.

Sorry for the low-information-density bug report, but I don't really
have time at the moment to debug failures in merge build test runs
because the queue of stuff still to merge is enormous...

thanks
-- PMM