Raise proper error when you try to load an empty qcow2 file

Fix #637

gns3server/modules/qemu/qcow2.py

@@ -62,7 +62,10 @@ class Qcow2:
         with open(self.path, 'rb') as f:
             content = f.read(struct.calcsize(struct_format))
 
-            self.magic, self.version, self.backing_file_offset, self.backing_file_size = struct.unpack_from(struct_format, content)
+            try:
+                self.magic, self.version, self.backing_file_offset, self.backing_file_size = struct.unpack_from(struct_format, content)
+            except struct.error:
+                raise Qcow2Error("Invalid file header for {}".format(self.path))
 
         if self.magic != 1363560955:  # The first 4 bytes contain the characters 'Q', 'F', 'I' followed by 0xfb.
             raise Qcow2Error("Invalid magic for {}".format(self.path))