xen: refactor debugtrace data

xen: refactor debugtrace data

As a preparation for per-cpu buffers do a little refactoring of the
debugtrace data: put the needed buffer admin data into the buffer as
it will be needed for each buffer. In order not to limit buffer size
switch the related fields from unsigned int to unsigned long, as on
huge machines with RAM in the TB range it might be interesting to
support buffers >4GB.

While at it switch debugtrace_send_to_console and debugtrace_used to
bool and delete an empty line.

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