virtio-mem: improve error message when unplug of device fails due to plugged memory

virtio-mem: improve error message when unplug of device fails due to plugged memory

The error message is actually expressive, considering QEMU only. But
when called from Libvirt, talking about "size" can be confusing, because
in Libvirt "size" translates to the memory backend size in QEMU (maximum
size) and "current" translates to the QEMU "size" property.

Let's simply avoid talking about the "size" property and spell out that
some device memory is still plugged.

Message-ID: <20240416141426.588544-1-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Cc: Liang Cong <lcong@redhat.com>
Cc: Mario Casquero <mcasquer@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>