tracetool: clarify that "formats" means "format strings"

The terminology used by tracetool is not consistent with C sprintf or
docs/devel/tracing.txt.  The word "formats" is sometimes used to mean
"format strings".

This patch clarifies comments and error messages that contain this word.

Note that the error message lines are longer than 80 characters but I
have not wrapped them to aid grepping.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180110202553.31889-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>