incusd/network: Optimize UsedByInstanceDevices #392

Job	Run time
Code	6m 51s
Client (1.20.x, ubuntu-latest)	17s
Documentation	5m 49s
System (1.20.x, cluster, dir)	12m 17s
Client (1.20.x, macos-latest)	56s
System (1.20.x, cluster, btrfs)	11m 18s
Client (1.20.x, windows-latest)	1m 46s
System (1.20.x, cluster, lvm)	12m 9s
System (1.20.x, cluster, zfs)	12m 30s
System (1.20.x, cluster, ceph)	15m 8s
System (1.20.x, cluster, random)	11m 29s
System (1.20.x, standalone, dir)	20m 30s
System (1.20.x, standalone, btrfs)	19m 56s
System (1.20.x, standalone, lvm)	36m 55s
System (1.20.x, standalone, zfs)	40m 31s
System (1.20.x, standalone, ceph)	1h 0m 38s
System (1.20.x, standalone, random)	22m 23s
System (stable, cluster, dir)	11m 46s
System (stable, standalone, dir)	20m 10s
System (tip, cluster, dir)	12m 8s
System (tip, standalone, dir)	20m 55s
	5h 56m 22s

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