tools/xenstore: Check the format printf for xprintf() and barf{,_perror}()

tools/xenstore: Check the format printf for xprintf() and barf{,_perror}()

Allow GCC to analyze the format printf for xprintf() and
barf{,_perror}().

Take the opportunity to define __noreturn to make the prototype for
barf{,_perror})() easier to read.

Also document why 'extern' is used for xprintf().

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>