With dev-libs/libxml2-2.11, xi:include errors out if it can't find a file, so add an empty fallback.
Used to work with dev-libs/libxml2-2.10.
Upstream has removed user_ and overview_ altogether in the next release as part of a larger docs change,
so this no longer applies.
See also: https://bugs.gentoo.org/906309
--- a/doc/manual/en_US/user_VBoxManage.xml
+++ b/doc/manual/en_US/user_VBoxManage.xml
@@ -242,107 +242,107 @@
on your particular host.
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Each time VBoxManage is invoked, only one