credit2: pack trace data better for xentrace_format

credit2: pack trace data better for xentrace_format

when tracing runstate changes, the vcpu and domain IDs
are encoded in the lower and higher, respectively, parts
of a 32 bits integer. When decoding a trace with
xentrace_format, this makes it possible to display
such events like this:

CPU0  833435853624 (+     768)  running_to_runnable [ dom:vcpu = 0x7fff0000 ]
CPU0  833435854416 (+     792)  runnable_to_running [ dom:vcpu = 0x00000007 ]

For consistency, we should do the same when displaying
the events coming from the Credit2 scheduler (when using
the same tool), and to do that, we need to invert the
order in which the fields are being put in the trace
struct right now.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>