hmp: fix block_set_io_throttle

Commit 7a9877a0 made the 'device' parameter to BlockIOThrottle
optional, favoring 'id' instead.  But it forgot to update the
HMP usage to set has_device, which makes all attempts to change
throttling via HMP fail with "Need exactly one of 'device' and 'id'"

CC: qemu-stable@nongnu.org
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20170120230359.4244-1-eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

hmp.c

@@ -1552,6 +1552,7 @@ void hmp_block_set_io_throttle(Monitor *mon, const QDict *qdict)
 {
     Error *err = NULL;
     BlockIOThrottle throttle = {
+        .has_device = true,
         .device = (char *) qdict_get_str(qdict, "device"),
         .bps = qdict_get_int(qdict, "bps"),
         .bps_rd = qdict_get_int(qdict, "bps_rd"),