gnu: QEMU: Adjust for Sphinx 4.0.

* gnu/packages/patches/qemu-sphinx-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/virtualization.scm (qemu)[source](patches): Add it.

gnu/local.mk

@@ -1634,6 +1634,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/qemu-build-info-manual.patch		\
   %D%/packages/patches/qemu-CVE-2021-20203.patch		\
   %D%/packages/patches/qemu-meson-compat.patch			\
+  %D%/packages/patches/qemu-sphinx-compat.patch			\
   %D%/packages/patches/qemu-glibc-2.27.patch 			\
   %D%/packages/patches/qemu-glibc-2.30.patch 			\
   %D%/packages/patches/qpdfview-qt515-compat.patch		\

gnu/packages/patches/qemu-sphinx-compat.patch

@@ -0,0 +1,18 @@
+Prevent Sphinx 4 and later from creating subdirectories, which
+the build scripts are not equipped to deal with.
+
+Taken from upstream:
+https://gitlab.com/qemu-project/qemu/-/issues/256
+https://gitlab.com/qemu-project/qemu/-/commit/269a7e97865cb863c9ca19e5f2e6a40ac9eddf82
+
+diff --git a/docs/conf.py b/docs/conf.py
+--- a/docs/conf.py
++++ b/docs/conf.py
+@@ -279,6 +279,7 @@
+      ['Stefan Hajnoczi <stefanha@redhat.com>',
+       'Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>'], 1),
+ ]
++man_make_section_directory = False
+ 
+ # -- Options for Texinfo output -------------------------------------------
+ 

gnu/packages/virtualization.scm

@@ -153,6 +153,7 @@ (define-public qemu
          "1f9hz8rf12jm8baa7kda34yl4hyl0xh0c4ap03krfjx23i3img47"))
        (patches (search-patches "qemu-CVE-2021-20203.patch"
                                 "qemu-meson-compat.patch"
+                                "qemu-sphinx-compat.patch"
                                 "qemu-build-info-manual.patch"))
        (modules '((guix build utils)))
        (snippet