small fix.  move up guid condition to avoid exception like:

ERROR: testListKernels (redhat_converter_test.GrubTest)
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Traceback (most recent call last):
  File "/home/cwolfe/guestconv/test/redhat_converter_test.py", line 37, in testListKernels
    self.img.inspect()
  File "/home/cwolfe/guestconv/test/test_helper.py", line 83, in inspect
    return self.converter.inspect()
  File "../guestconv/converter.py", line 152, in inspect
    (root_bl, root_info, root_devices) = converter.inspect()
  File "../guestconv/converters/redhat.py", line 49, in inspect
    self._bootloader = self._inspect_bootloader()
  File "../guestconv/converters/redhat.py", line 58, in _inspect_bootloader
    return bl(self._h, self._root, self._logger)
  File "../guestconv/converters/grub.py", line 242, in __init__
    if guid == u'C12A7328-F81F-11D2-BA4B-00A0C93EC93B':
UnboundLocalError: local variable 'guid' referenced before assignment

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