virtio-mem: default-enable "dynamic-memslots"

JIRA: https://issues.redhat.com/browse/RHEL-24045 Upstream: RHEL only

We only support selected vhost-user devices in combination with virtio-mem in RHEL. One devices that works well is virtiofsd, devices that are currently incompatible include DPDK and SPDK.

The vhost devices we support must be compatible with the dynamic-memslot feature (i.e., support at least 509 memslots, support dynamically adding/ removing memslots), such that setting "dynamic-memslots=on" will work a expected and not make certain QEMU commandlines or hotplug of vhost-user devices bail out.

Let's set "dynamic-memslots=on" starting with RHEL 9.4, so we get the benefits (i.e., reduced metadata consumption in KVM, majority of unplugged memory being inaccessible) as default.

When wanting to run virtio-mem with incompatible vhost-user devices, it might just work (if the vhost-user device is created before the virtio-mem device), or the feature can be manually disabled by specifying "dynamic-memslots=off".

Signed-off-by: David Hildenbrand david@redhat.com