qemu-iotests: add 202 external snapshots IOThread test

QMP 'transaction' blockdev-snapshot-sync with multiple disks in an
IOThread is an untested code path.  Several bugs have been found in
connection with this command.  This patch adds a test case to prevent
future regressions.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171206144550.22295-10-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
