AccelClass: Introduce accel_setup_post

This is called just before os_setup_post.  Currently none of the
accelerators provide this hook, but the Xen one is going to provide
one in a moment.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>

vl.c

@@ -4742,6 +4742,7 @@ int main(int argc, char **argv, char **envp)
         vm_start();
     }
 
+    accel_setup_post(current_machine);
     os_setup_post();
 
     main_loop();