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: NFabien Chouteau <chouteau@adacore.com>
Acked-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>