trace-cmd library man pages: Extend the examples of reading host-guest trace data

The trace-cmd library supports reading trace data from a host and
multiple guest machines as a single trace session, if they are recorded
at the same time. Added more relevant example in the man page, showing
that use case.

Link: https://lore.kernel.org/linux-trace-devel/20210121103605.224108-5-tz.stoyanov@gmail.com

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>