qemu-iotests/087: Avoid blockdev-add with id

qemu-iotests/087: 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.

The test cases that test conflicts between the 'id' option to
blockdev-add and existing block devices or the 'node-name' of the same
command can be removed because it won't be possible to specify this at
the end of the series.

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