Switch qemu to overlay convention

.orchestra/config/components.yml

 #@ load("@ytt:template", "template")
 #@ load("@ytt:data", "data")
 
-#@ load("/components/qemu.lib.yml", "qemu_component")
 #@ load("/components/revng.lib.yml", "revng_component")
 #@ load("/components/revng_c.lib.yml", "revng_c_component")
 #@ load("/components/revng_qa.lib.yml", "revng_qa_component")
@@ -32,7 +31,6 @@
 
 #@ load("/global_options.lib.yml", "options")
 components:
-  qemu: #@ qemu_component
   revng: #@ revng_component
   revng-c: #@ revng_c_component
   revng-qa: #@ revng_qa_component

.orchestra/config/components/qemu.lib.yml → .orchestra/config/components/qemu.yml

+#@ load("@ytt:overlay", "overlay")
 #@ load("/lib/make.lib.yml", "make")
 #@ load("/lib/optimization_flavors.lib.yml", "typical_project_flavors")
 
@@ -49,5 +50,8 @@ builds:
   #@ end
 #@ end
 
+#@overlay/match by=overlay.all, expects=1
+#@overlay/match-child-defaults missing_ok=True
 ---
-#@ qemu_component = _qemu_component()
+components:
+  qemu: #@ _qemu_component()