torture: Consistently name "qemu*" test output files

There is "qemu-affinity", "qemu-cmd", "qemu-retval", but also "qemu_pid".
This is hard to remember, not so good for bash tab completion, and just
plain inconsistent.  This commit therefore renames the "qemu_pid" file to
"qemu-pid".  A couple of the scripts must deal with old runs, and thus
must handle both "qemu_pid" and "qemu-pid", but new runs will produce
"qemu-pid".

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>