tests: Pass literal format strings directly to qmp_FOO()

tests: Pass literal format strings directly to qmp_FOO()

The qmp_FOO() take a printf-like format string.  In a few places, we
assign a string literal to a variable and pass that instead of simply
passing the literal.  Clean that up.

Bonus: gets rid of non-literal format strings.  A step towards
compile-time format string checking without triggering
-Wformat-nonliteral.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-12-armbru@redhat.com>