trace: enable tracing in qemu-nbd

trace: enable tracing in qemu-nbd

Please note, trace_init_backends() must be called in the final process,
i.e. after daemonization. This is necessary to keep tracing thread in
the proper process.

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1466174654[email protected]
CC: Paolo Bonzini <[email protected]>
CC: Kevin Wolf <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>