virtcontainers: store: Add a ItemLock API

The ItemLock API allows for taking shared and exclusive locks on all
items.
For virtcontainers, this is specialized into taking locks on the Lock
item, and will be used for sandbox locking.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

virtcontainers/store/manager.go

@@ -261,3 +261,13 @@ func (s *Store) Raw(id string) (string, error) {
 
 	return s.backend.raw(id)
 }
+
+// ItemLock takes a lock on an item.
+func (s *Store) ItemLock(item Item, exclusive bool) (string, error) {
+	return s.backend.lock(item, exclusive)
+}
+
+// ItemUnlock unlocks an item.
+func (s *Store) ItemUnlock(item Item, token string) error {
+	return s.backend.unlock(item, token)
+}