libqtest: Clean up how we read the QMP greeting

libqtest: Clean up how we read the QMP greeting

qtest_init() still uses the qtest_qmp_discard_response(s, "") hack to
receive the greeting, even though we have qtest_qmp_receive() since
commit 66e0c7b187e.  Put it to use.

Bonus: gets rid of an empty format string.  A step towards
compile-time format string checking without triggering
-Wformat-zero-length.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180806065344[email protected]>