incusd/instance/qemu: Fix shutdown race #676

Job	Run time
Documentation	5m 9s
Code (oldstable)	4m 19s
Client (oldstable, ubuntu-latest)	1m 25s
Code (stable)	4m 16s
System (stable, cluster, dir)	8m 43s
Client (oldstable, macos-latest)	1m 11s
Code (tip)	6m 57s
System (stable, cluster, btrfs)	8m 44s
Client (oldstable, windows-latest)	1m 38s
System (stable, cluster, lvm)	8m 43s
Client (stable, ubuntu-latest)	1m 33s
System (stable, cluster, zfs)	8m 44s
Client (stable, macos-latest)	1m 16s
System (stable, cluster, ceph)	8m 43s
Client (stable, windows-latest)	1m 38s
System (stable, cluster, random)	8m 43s
System (stable, standalone, dir)	8m 42s
System (stable, standalone, btrfs)	8m 43s
System (stable, standalone, lvm)	8m 43s
System (stable, standalone, zfs)	8m 44s
System (stable, standalone, ceph)	8m 42s
System (stable, standalone, random)	8m 42s
System (oldstable, cluster, dir)	8m 46s
System (oldstable, standalone, dir)	8m 43s
System (tip, cluster, dir)	8m 42s
System (tip, standalone, dir)	8m 44s
	2h 48m 53s

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