* Replace "available only" by the more common "only available".

* Tracing options depend on the configuration of the QEMU executable,
  so clarify the help text for both options.

Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>