add cache size

vm.py

@@ -225,6 +225,7 @@ class VMDisk:
     driver_type = None
     driver_cache = None
     target_device = None
+    cache_size = None
 
     def __init__(self,
                  name,
@@ -235,7 +236,8 @@ class VMDisk:
                  driver_type="qcow2",
                  driver_cache="none",
                  target_device="vda",
-                 target_bus="virtio"):
+                 target_bus="virtio",
+                 cache_size=1024):
         self.name = name
         self.source = source
         self.disk_type = disk_type
@@ -245,6 +247,7 @@ class VMDisk:
         self.driver_cache = driver_cache
         self.target_device = target_device
         self.target_bus = target_bus
+        self.cache_size = cache_size
 
     @classmethod
     def deserialize(cls, desc):
@@ -259,11 +262,14 @@ class VMDisk:
         ET.SubElement(xml_top, 'target',
                       attrib={'dev': self.target_device,
                               'bus': self.target_bus})
-        ET.SubElement(xml_top, 'driver',
+        driver = ET.SubElement(xml_top, 'driver',
                       attrib={
                           'name': self.driver_name,
                           'type': self.driver_type,
                           'cache': self.driver_cache})
+        metadata_cache = ET.SubElement(driver, 'metadata_cache')
+        ET.SubElement(metadata_cache, 'max_size', 
+                        attrib={'unit': 'bytes'}).text = str(self.cache_size)
         return xml_top
 
     def dump_xml(self):