This backend sends trace events to standard error output during the emulation.

Also add a "--list-backends" option to tracetool, so configure script can
display the list of available backends.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Acked-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>