xend: reinstate XendOptionsFile.get_external_migration_tool

This was removed by 26167:31dcc0e08754 "xend: Remove old vtpm support
from xm" but at least one caller was left. Reinstate with
s/TPM/device/ since it appears that this functionality could apply
elsewhere.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

tools/python/xen/xend/XendOptions.py

@@ -349,6 +349,10 @@ class XendOptions:
         else:
             return None
 
+    def get_external_migration_tool(self):
+        """@return the name of the tool to handle virtual device migration."""
+        return self.get_config_string('external-migration-tool', self.external_migration_tool_default)
+
     def get_enable_dump(self):
         return self.get_config_bool('enable-dump', 'no')
 

tools/python/xen/xend/tests/xend-config.sxp

@@ -127,3 +127,5 @@
 # Whether to enable core-dumps when domains crash.
 #(enable-dump no)
 
+# The tool used for initiating virtual device migration
+#(external-migration-tool '')