The buffer was being allocated of size string length plus two.
Around the string two quotes were being added, but no terminating NUL.
It was then compared using g_assert_cmpstr(), resulting in fairly random
assertion failures:

 ERROR:tests/test-string-output-visitor.c:213:test_visitor_out_enum: assertion failed (str == str_human): ("\"value1\"" == "\"value1\"\001EEEEEEEEEEEEEE\0171")

There is no g_assert_cmpnstr() counterpart, so use g_strdup_printf()
for safely assembling the string in the first place.

Cc: Hu Tao <hutao@cn.fujitsu.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Suggested-by: Eric Blake <eblake@redhat.com>
Fixes: b4900c0e tests: add human format test for string output visitor
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>