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: Eric Blake <eblake@redhat.com>
Message-Id: <20170120230359.4244-1-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>