Dataset accounting: VMM zone filesystems should be explicitly requested

[smklein]

Currently, the sled agent picks a random U.2 to hold the zone filesystem for instances:

omicron/sled-agent/src/instance.rs

Lines 1376 to 1385 in 89b998c

	let latest_disks = self
	.storage
	.get_latest_disks()
	.await
	.all_u2_mountpoints(ZONE_DATASET);
	let root = latest_disks
	.into_iter()
	.choose(&mut rng)
	.ok_or_else(|| Error::U2NotFound)?;

This, unfortunately, leaves nexus out-of-the-loop regarding space allocation within the zone dataset.

Nexus should be aware of where instances are provisioned to manage accounting explicitly.

[smklein]

See also: #6109