Detect UTF-16 encoded XML files

This code doesn't support leading blank characters, since it's
invalid, and the filter doesn't handle it anyway.

Change-Id: I494e9f85351539d27577dc7df8be420c0c66570e
Reviewed-on: https://gerrit.libreoffice.org/7204
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>