virtio-mem: Skip most of virtio_mem_unplug_all() without plugged memory

virtio-mem: Skip most of virtio_mem_unplug_all() without plugged memory

Already when starting QEMU we perform one system reset that ends up
triggering virtio_mem_unplug_all() with no actual memory plugged yet.
That, in turn will trigger ram_block_discard_range() and perform some
other actions that are not required in that case.

Let's optimize virtio_mem_unplug_all() for the case that no memory is
plugged. This will be beneficial for x-ignore-shared support as well.

Message-ID: <20230706075612.67404-3-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>