Dynamical `Host.evacuate` batch size about xen-api

[lindig]

In general I'm fine with the proposal when it comes with a sensible default that we are testing.

[olivierlambert]

To give you more context on the functional side of things, we had some reports of users having a migration time/pause in their guests longer than they would like, when migrated 10 by 10. Migrating 3 by 3 (or even 2 by 2) is solving entirely the issue.

So instead of changing the value in all hosts (which is done as a workaround), we thought that adding an extra parameter that could be changed more easily (ie via Xen Orchestra) would be a better approach, hence this PR :) On my side, I'm thinking on getting XO calling evacuate with 2 or 3 by default, but make it configurable somewhere in the software, without requiring any host modification (and that's a big plus).