tools/virtio: fix typo in README instructions

We need to have a unique chardev for each data path, else the chardevs
will collide and qemu will die with this message:

  qemu-system-x86_64: -device
  virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel0,
  id=channel1,name=trace-path-cpu0:
  Property 'virtserialport.chardev' can't take value 'charchannel0':
  Device 'charchannel0' is in use

Signed-off-by: Ross Zwisler <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>

Author: Ross Zwisler

tools/virtio/virtio-trace/README

@@ -61,7 +61,7 @@ and
       id=channel0,name=agent-ctl-path\
  ##data path##
      -chardev pipe,id=charchannel1,path=/tmp/virtio-trace/trace-path-cpu0\
-     -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel0,\
+     -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,\
       id=channel1,name=trace-path-cpu0\
       ...