Add tag cache for VMs.

Indicates if driver used for disk backing is using some sort of caching which
may lead to data corruption for a live migration.

cloudcontrol/node/hypervisor/domains/__init__.py

@@ -44,6 +44,9 @@ class VirtualMachine(object):
         #: tags for this VM
         # FIXME use a tag db instance
         self.tags = dict((t.name, t) for t in tag_inspector(vm_tags, self))
+        #: Driver cache behavior for each VM storage, see
+        #: http://libvirt.org/formatdomain.html#elementsDisks
+        self.cache_behaviour = dict()
         # define dynamic tags
         i = 0
         for v in self.iter_disks():
@@ -52,6 +55,8 @@ class VirtualMachine(object):
                 Tag('disk%s_path' % i, v.path, 10),
                 Tag('disk%s_pool' % i, v.storage, 10),  # FIXME: change
                 Tag('disk%s_vol' % i, v.name, 10),
+                Tag('disk%s_cache' %i,
+                    lambda: self.cache_behaviour.get(v.path), 10)
             ):
                 self.tags[t.name] = t
 
@@ -129,6 +134,13 @@ class VirtualMachine(object):
                 continue
 
             path = d.find('source').get(dict(file='file', block='dev')[type_])
+
+            # update cache behaviour
+            driver = d.find('driver')
+            if driver is None:
+                driver = {}
+            self.cache_behaviour[path] = driver.get('cache', 'default')
+
             volume = self.hypervisor.storage.get_volume(path)
             if volume is None:
                 continue