Oxenstored does not implement `quota_set` control commands

[last-genius]

See do_control_quota in control.c in upstream Xen repo.

These calls could be useful for dynamic adjustment of quotas per-domain by the toolstack - though not clear how problematic the default quotas could be in the first place.

[last-genius]

Note that this would require refactoring the quota system to have per-domain limits, as currently there's a single Quota.t per Store.t