qemu-options: Improve help texts for options which depend on configure * 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: Stefan Weil <weil@mail.berlios.de> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>