docs: set CONFDIR when running sphinx

The default configuration path /etc/qemu can be overriden with configure
options, and the generated documentation used to reflect it.

Fixes regression introduced in commit
f8aa24ea9a82da38370470c6bc0eaa393999edfe ("meson: sphinx-build").

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1902537
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20201201183704.299697-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/meson.build

@@ -9,7 +9,7 @@ endif
 # Check if tools are available to build documentation.
 build_docs = false
 if sphinx_build.found()
-  SPHINX_ARGS = [sphinx_build]
+  SPHINX_ARGS = ['env', 'CONFDIR=' + qemu_confdir, sphinx_build]
   # If we're making warnings fatal, apply this to Sphinx runs as well
   if get_option('werror')
     SPHINX_ARGS += [ '-W' ]