libqtest: Replace qtest_startf() by qtest_initf()

libqtest: Replace qtest_startf() by qtest_initf()

qtest_init() creates a new QTestState, and leaves @global_qtest alone.
qtest_start() additionally assigns it to @global_qtest, but
qtest_startf() additionally assigns NULL to @global_qtest.  This makes
no sense.  Replace it by qtest_initf() that works like qtest_init(),
i.e. leaves @global_qtest alone.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-23-armbru@redhat.com>