Convert attribute value to UTF-8 when passing it to libxml2

Convert attribute value to UTF-8 when passing it to libxml2

Using toUtf8, requiring the OUString to actually contain well-formed data, but
which is likely OK for this test-code--only function, and is also what similar
dumpAsXml functions e.g. in editeng/source/items/textitem.cxx already use.

This appears to have been broken ever since the code's introduction in
553f10c71a2cc92f5f5890e24948f5277e3d2758 "add dumpAsXml() to more pool items",
and it would typically only have written the leading zero or one
(depending on the architecture's endianness) characters.  (I ran across it on
big-endian s390x, where CppunitTest_sd_tiledrendering
SdTiledRenderingTest::testTdf104405 failed because of

> Entity: line 2: parser error : Input is not proper UTF-8, indicate encoding !
> Bytes: 0xCF 0x22 0x2F 0x3E
> ation=""/><SfxPoolItem whichId="4017" typeName="13SvxBulletItem" presentation="%
>                                                                                ^

apparently reported from within libxml2.)

Change-Id: I4b116d3be84098bd8b8a13b6937da70a1ee02c7f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103236
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins
Gbp-Pq: Name bigendian.diff