xentrace: Don't append trace on existing file.

xentrace: Don't append trace on existing file.

When you run "xentrace -e <mask> trace.output" the first time, all is
fine. When you run it a second time, then the data is appended which
makes you reading old data with xentrace_format and you interprete it
as new data. This usually happens when you automated tracing guests
with xentrace and xentrace_format with a script.

Therefore, attached patch makes xentrace to truncate the file to zero
bytes before writing any data.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>