libvirt-glib fails to cross build from source, because meson.build
disables gobject-introspection for cross compilation and then fails as
vala support requires gobject-introspection. On Debian,
gobject-introspection actually works due to a cross wrapper doing the
heavy lifting. Please lift the restriction. Once doing so, the build
fails via #1126007. I'm attaching a patch for your convenience.

Helmut