xend: Unbreak live migration with tapdisk2 after 20691:054042ba73b6

vm.image does not exist at this point in the restore process.
I haven't looked at the memory_sharing code. It's likely something
better is needed to make that work across relocation.

Signed-off-by: Brendan Cully <brendan@cs.ubc.ca>

tools/python/xen/xend/server/BlktapController.py

@@ -198,7 +198,7 @@ class Blktap2Controller(BlktapController):
                     self.deviceClass = 'tap2'
                     return devid
 
-        if self.vm.image.memory_sharing:
+        if self.vm.image and self.vm.image.memory_sharing:
             cmd = [ TAPDISK_BINARY, '-n', '%s:%s' % (params, file), '-s', '%d' % self.vm.getDomid() ]
         else:
             cmd = [ TAPDISK_BINARY, '-n', '%s:%s' % (params, file) ]