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 <zwisler@google.com>
Message-Id: <20230215223350.2658616-7-zwisler@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>