doc: Avoid orphan node.

Commit 4b5a6fbc turned "Transparent
Emulation with QEMU" into a node, but that was an orphan node, with no
up/next/previous links.

* doc/guix.texi (Virtualization Services): Capitalize subsection title.
(Transparent Emulation with QEMU): Remove "@node" and add "@anchor".
(Daemon Offload Setup): Adjust cross-reference accordingly.