virtio-mem: check if the config changed before fake offlining memory

If we repeatedly fail to fake offline memory to unplug it, we won't be
sending any unplug requests to the device. However, we only check if the
config changed when sending such (un)plug requests.

We could end up trying for a long time to unplug memory, even though
the config changed already and we're not supposed to unplug memory
anymore. For example, the hypervisor might detect a low-memory situation
while unplugging memory and decide to replug some memory. Continuing
trying to unplug memory in that case can be problematic.

So let's check on a more regular basis.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230713145551.2824980-5-david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>