qemu-iotests: Extend non-shared storage migration test (194)

This is the follow-up patch that was discussed[*] as part of feedback to
qemu-iotest 194.

Changes in this patch:

  - Supply 'job-id' parameter to `drive-mirror` invocation.

  - Once migration completes, issue QMP `block-job-cancel` command on
    the source QEMU to gracefully complete `drive-mirror` operation.

  - Once the BLOCK_JOB_COMPLETED event is emitted, stop the NBD server
    on the destination QEMU.

  - Check for both the events: MIGRATION and BLOCK_JOB_COMPLETED.

With the above, the test will also be (almost) in sync with the
procedure outlined in the document 'live-block-operations.rst'[+]
(section: "QMP invocation for live storage migration with
``drive-mirror`` + NBD").

[*] https://lists.nongnu.org/archive/html/qemu-devel/2017-08/msg04820.html
    -- qemu-iotests: add 194 non-shared storage migration test
[+] https://git.qemu.org/gitweb.cgi?p=qemu.git;a=blob;f=docs/interop/live-block-operations.rst

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20170829165058.8229-1-kchamart@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
