[cmake] Add apple backend

Author: sharder996

CMakeLists.txt

@@ -187,6 +187,10 @@ if(UNIX)
   endif()
 
   list(APPEND MULTIPASS_BACKENDS qemu)
+
+  if(APPLEVZ_ENABLED)
+    list(APPEND MULTIPASS_BACKENDS apple)
+  endif()
 endif()
 
 # Boost config