doc/sphinx/hxtool.py: add optional label argument to SRST directive

We can't just embed labels directly into files like qemu-options.hx which
are included from multiple top-level rST files, because Sphinx sees the
labels as duplicate: https://github.com/sphinx-doc/sphinx/issues/9707

So add an optional argument to the SRST directive which causes a label
of the form '.. _DOCNAME-HXFILE-LABEL:' to be emitted, where 'DOCNAME'
is the name of the top level rST file, 'HXFILE' is the filename of the
.hx file, and 'LABEL' is the text provided within the 'SRST()' directive.
Using the DOCNAME of the top-level rST document means that it is unique
even when the .hx file is included from two different documents, as is
the case for qemu-options.hx

Now where the Xen PV documentation refers to the documentation for the
-initrd command line option, it can emit a link directly to it as
'<system/invocation-qemu-options-initrd>'.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240130190348.682912-1-dwmw2@infradead.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/i386/xen.rst

@@ -132,7 +132,8 @@ The example above provides the guest kernel command line after a separator
 (" ``--`` ") on the Xen command line, and does not provide the guest kernel
 with an actual initramfs, which would need to listed as a second multiboot
 module. For more complicated alternatives, see the command line
-documentation for the ``-initrd`` option.
+:ref:`documentation <system/invocation-qemu-options-initrd>` for the
+``-initrd`` option.
 
 Host OS requirements
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