Modify whole logic of get_ovf_and_root_vmdk

As the current method does not work and we need to care about
blocking caused by file.read() without an argument, the following
modifications were done:

* give a chunck size to read():
    so that operation does not block nor wait until a whole file
    is downloaded (not sure but python should work this way)

* set a proper chunk size:
    to avoid error caused by unit data (used for size) handled by
    socket library (I have got error for this with 10Gib vmdk)
    To be on the safe side, chunk should be less than this size.

***Warning***
   There could be a possibility of MemoryError for huge vmdk as
   io.BytesIO is used for return value.