qemu-iotests: add 194 non-shared storage migration test

Non-shared storage migration with NBD and drive-mirror is currently not
tested by qemu-iotests.  This test case covers the basic migration
scenario.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Based-on: <20170823134242.12080-1-famz@redhat.com>
Message-Id: <20170823140506.28723-1-stefanha@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>