tests/drive_del-test: Fix harmless JSON interpolation bug

tests/drive_del-test: Fix harmless JSON interpolation bug

test_after_failed_device_add() does this:

    response = qmp("{'execute': 'device_add',"
                   " 'arguments': {"
                   "   'driver': 'virtio-blk-%s',"
                   "   'drive': 'drive0'"
                   "}}", qvirtio_get_dev_type());

Wrong.  An interpolation specification must be a JSON token, it
doesn't work within JSON string tokens.  The code above doesn't use
the value of qvirtio_get_dev_type(), and sends arguments

    {"driver": "virtio-blk-%s", "drive": "drive0"}}

The command fails because there is no driver named "virtio-blk-%".
Harmless, since the test wants the command to fail.  Screwed up in
commit 2f84a92ec63.

Fix the obvious way.  The command now fails because the drive is
empty, like it did before commit 2f84a92ec63.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180823164025[email protected]>