virtio-mem: Drop manual check for already present memory

Registering our parent resource will fail if any memory is still present
(e.g., because somebody unloaded the driver and tries to reload it). No
need for the manual check.

Move our "unplug all" handling to after registering the resource.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-13-david@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>