Fix for device name detection

Fix for device name detection

Our nbd devices end up in /dev/block/nbd*. check_conn() tries to
derive the path to the pid file under /sys/block/<nbd device>/pid by
stripping a leading /dev/ path prefix. In our case it ends up with
/sys/block/block/<nbd device>/pid and is unable to locate the pid file.
The attached patch nbd-client-devname.patch uses

        if( (p=strrchr(devname, '/')) ) {
                devname=p+1;
        }

to derive the name of the device.