incusd/instances/qemu: Don't fail event sending on missing agent #306

Job	Run time
Code	7m 14s
Documentation	6m 41s
Client (1.20.x, ubuntu-latest)	1m 19s
System (1.20.x, cluster, dir)	12m 20s
Client (1.20.x, macos-latest)	1m 42s
System (1.20.x, cluster, btrfs)	13m 10s
Client (1.20.x, windows-latest)	2m 55s
System (1.20.x, cluster, lvm)	12m 59s
System (1.20.x, cluster, zfs)	14m 27s
System (1.20.x, cluster, ceph)	16m 21s
System (1.20.x, cluster, random)	11m 55s
System (1.20.x, standalone, dir)	23m 38s
System (1.20.x, standalone, btrfs)	23m 11s
System (1.20.x, standalone, lvm)	40m 3s
System (1.20.x, standalone, zfs)	42m 36s
System (1.20.x, standalone, ceph)	1h 14m 31s
System (1.20.x, standalone, random)	31m 9s
System (stable, cluster, dir)	12m 4s
System (stable, standalone, dir)	21m 56s
System (tip, cluster, dir)	13m 0s
System (tip, standalone, dir)	22m 7s
	6h 45m 18s

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