incusd/instance/qemu: Cap hotplug CPU slots to 64 #419

Job	Run time
Code	3m 53s
Documentation	6m 45s
Client (1.21.x, ubuntu-latest)	1m 3s
Client (1.21.x, macos-latest)	1m 26s
System (1.21.x, cluster, dir)	12m 14s
Client (1.21.x, windows-latest)	2m 22s
System (1.21.x, cluster, btrfs)	11m 20s
System (1.21.x, cluster, lvm)	13m 47s
System (1.21.x, cluster, zfs)	13m 27s
System (1.21.x, cluster, ceph)	17m 32s
System (1.21.x, cluster, random)	11m 7s
System (1.21.x, standalone, dir)	21m 15s
System (1.21.x, standalone, btrfs)	20m 34s
System (1.21.x, standalone, lvm)	42m 59s
System (1.21.x, standalone, zfs)	41m 27s
System (1.21.x, standalone, ceph)	42m 59s
System (1.21.x, standalone, random)	30m 8s
System (stable, cluster, dir)	11m 52s
System (stable, standalone, dir)	21m 10s
System (tip, cluster, dir)	4m 52s
System (tip, standalone, dir)	3m 9s
	5h 35m 21s

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