Should shadow_lock be spin_lock_recursive?

[Arun Sharma <arun.sharma@intel.com>]

During our testing, we found this code path where xen attempts to grab 
the shadow_lock, while holding it - leading to a deadlock.

 >> free_dom_mem->
 >> shadow_sync_and_drop_references->
 >> shadow_lock -> ..................... first lock
 >> shadow_remove_all_access->
 >> remove_all_access_in_page->
 >> put_page->
 >> free_domheap_pages->
 >> shadow_drop_references->
 >> shadow_lock -> ..................... second lock

Questions:

- should shadow lock be recursive?
- is shadow lock too coarse grained? It seems to have led to a lot of 
code refactoring (__foo without lock and foo with lock). But there may 
be more such instances we haven't found yet.

	-Arun