fix(vmm): skip discarding a virtio-mem range with nothing plugged

handle_unplug_all_request builds a range covering the whole hotpluggable
region and passed it to update_range unconditionally, so update_range
ran a full-region discard on every UNPLUG_ALL even when no block in the
range was plugged, repeating the same work on every such request.

Return early from update_range when unplugging a range that has no
plugged block: the bitmap, KVM slots and memory are already in the
requested state. update_slot's own `prev == plug` guard already makes
the per-slot KVM work a no-op, so this only drops the redundant discard.

Signed-off-by: Jay Chung <jaehoc@amazon.com>