Commit 934659c4 switched the iotests to run qemu and qemu-nbd from a bash
subshell, in order to catch segfaults.  Unfortunately, this means the
process PID cannot be captured via '$!'. We stopped killing qemu and
qemu-nbd processes, leaving a lot of orphaned, running qemu processes
after executing iotests.

Since the process is using exec in the subshell, the PID is the
same as the subshell PID.

Track these PIDs for cleanup using pidfiles in the $TEST_DIR. Only
track the qemu PID, however, if requested - not all usage requires
killing the process.
Reported-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>
Message-id: 9e4f958b3895b7259b98d845bb46f000ba362869.1446232490.git.jcody@redhat.com
[mreitz@redhat.com: Replaced '! -z "..."' by '-n "..."']
Signed-off-by: NMax Reitz <mreitz@redhat.com>