incusd/instance/qemu/agent-loader: Handle legacy lxd-agent-loader #405

Job	Run time
Client (1.20.x, ubuntu-latest)	26s
Code	7m 31s
Documentation	7m 9s
Client (1.20.x, macos-latest)	30s
System (1.20.x, cluster, dir)	11m 30s
Client (1.20.x, windows-latest)	3m 0s
System (1.20.x, cluster, btrfs)	11m 35s
System (1.20.x, cluster, lvm)	12m 49s
System (1.20.x, cluster, zfs)	12m 41s
System (1.20.x, cluster, ceph)	16m 24s
System (1.20.x, cluster, random)	13m 33s
System (1.20.x, standalone, dir)	22m 23s
System (1.20.x, standalone, btrfs)	22m 27s
System (1.20.x, standalone, lvm)	39m 45s
System (1.20.x, standalone, zfs)	28m 10s
System (1.20.x, standalone, ceph)	1h 17m 35s
System (1.20.x, standalone, random)	22m 29s
System (stable, cluster, dir)	10m 31s
System (stable, standalone, dir)	22m 34s
System (tip, cluster, dir)	11m 25s
System (tip, standalone, dir)	21m 52s
	6h 16m 19s

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