perftune.py: support kvm-clock on tune-clock
perftune.py: support kvm-clock on tune-clock

On KVM guest, optimal clocksource should be 'kvm-clock' instead of 'tsc'.
This adds 'kvm' as another architecture, and set preferred clocksource to 'kvm-clock'.
This is also needed for Amazon Nitro VMs, since it is KVM based VM.

Related scylladb/scylla#7444

Closes #826

Reviewed-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>

dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar