xen: fix debugtrace clearing

xen: fix debugtrace clearing

After dumping the debugtrace buffer it is cleared. This results in some
entries not being printed in case the buffer is dumped again before
having wrapped.

While at it remove the trailing zero byte in the buffer as it is no
longer needed. Commit b5e6e1ee8da59f introduced passing the number of
chars to be printed in the related interfaces, so the trailing 0 byte
is no longer required.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>