qemu-iotests: Allow starting new qemu after cleanup

qemu-iotests: Allow starting new qemu after cleanup

After _cleanup_qemu(), test cases should be able to start the next qemu
process and call _cleanup_qemu() for that one as well. For this to work
cleanly, we need to improve the cleanup so that the second invocation
doesn't try to kill the qemu instances from the first invocation a
second time (which would result in error messages).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>