incusd/instance/lxc: Detect bad idmap and find new one #331

Job	Run time
Documentation	6m 21s
Client (1.20.x, ubuntu-latest)	1m 5s
System (1.20.x, cluster, dir)	11m 14s
Code	6m 44s
Client (1.20.x, macos-latest)	1m 28s
System (1.20.x, cluster, btrfs)	11m 18s
Client (1.20.x, windows-latest)	2m 21s
System (1.20.x, cluster, lvm)	13m 49s
System (1.20.x, cluster, zfs)	12m 53s
System (1.20.x, cluster, ceph)	17m 18s
System (1.20.x, cluster, random)	12m 32s
System (1.20.x, standalone, dir)	21m 57s
System (1.20.x, standalone, btrfs)	23m 31s
System (1.20.x, standalone, lvm)	40m 39s
System (1.20.x, standalone, zfs)	42m 59s
System (1.20.x, standalone, ceph)	1h 13m 48s
System (1.20.x, standalone, random)	22m 27s
System (stable, cluster, dir)	9m 20s
System (stable, standalone, dir)	3m 5s
System (tip, cluster, dir)	11m 7s
System (tip, standalone, dir)	22m 4s
	6h 8m 0s

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