virtio pmem: fix wrong mem region condition

Coverity reported memory region returns zero
for non-null value. This is because of wrong
arguments to '?:' , fixing this.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Message-Id: <20190712073554.21918-2-pagupta@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>