sim engine allows creating two pools w/ shared blockdevs

[mulkieran]

the strat engine doesn't avoid this by checking its data structures, it avoids it by checking the signatures on the block devices it looks at.

This is a bit of a delicate question. Should the real engine check data structures, or should it just rely on data off blockdevs passed. These could contradict each other.

[jbaublitz]

@bmr-cymru Per your earlier question about this, it looks like we have an issue out for this.