hbal: Implement grouping of moves into jobsets

Since moving two instances between different node-quadruples (inst X: A,
B → C, D and inst Y: E, F → G, H) can be parallelised by Ganeti, it
makes sense to split the operation list into jobsets whose execution
must be serialised, but whose individual jobs can be parallelised.

Of course, this doesn't help, because today a single logical job is for
some cases split in multiple gnt-instance invocations, which (if run
using ‘--submit’) might be wrongly parallelised by Ganeti. So while for
now this is mostly a cosmetic hint to the user, and it just a step for
future improvements.