xl: optionally print timestamps when running xl commands

xl: optionally print timestamps when running xl commands

Add a global option "-T" to xl to enable timestamps in the output from
libxl and libxc. This is most useful with long running commands such
as "migrate".

During 'xl -v.. migrate domU host' a large amount of debug is generated.
It is difficult to map each line to the sending and receiving side.
Also the time spent for migration is not reported.

With 'xl -T migrate domU host' both sides will print timestamps and
also the pid of the invoked xl process to make it more obvious which
side produced a given log line.

Note: depending on the command, xl itself also produces other output
which does not go through libxentoollog. As a result such output will
not have timestamps prepended.

This change adds also the missing "-t" flag to "xl help" output.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
Reviewed-by: Ian Jackson <iwj@xenproject.org>