cpu config: Unify the default value for 'kvm'

cpu config: Unify the default value for 'kvm'

Make the default value for 'kvm' consistent, taking into account
whether the VM will run on the same CPU architecture as the host.

This would be a breaking change to CPU hotplug for VMs with a
different CPU architecture running on an x86_64 host, as in this case
the default CPU type for CPU hotplug changes from 'kvm64' to 'qemu64'.
However, CPU hotplug of non x86_64 architectures is not supported
anyway, so this is not a breaking change after all.

It should be noted that this change does alter the CPU hotplug
behaviour when emulating an x86_64 CPU on a non-x86_64 host. This is
however not officially supported in Proxmox VE.

Signed-off-by: Filip Schauer <f.schauer@proxmox.com>