qemu-iotests/139: Avoid blockdev-add with id

We want to remove the 'id' option for blockdev-add. This removes one
user of the option and makes it use only node names.

Some test cases that used to work with an unattached BlockBackend are
removed, either because they don't make sense with an attached device or
because the equivalent test case with an attached device already exists.

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