Check usage of real time features of qemu

We run all VMs with -realtime mlock=off, check if that make sense, especially in context of low latency. From the first reading of docs, we should enable it in some cases (running tests).

We could consider adding to VMs used for tests, something like that:

<domain>
<memoryBacking>
<locked/>
</memoryBacking>
<memtune>
<hard_limit unit='G'>5</hard_limit>
</memtune>
</domain>

More resources: https://libvirt.org/formatdomain.html