Supporting functionalities for direct migration

[matt-tsai]

Main changes include: ReadWrite interface for transport TPMs, adding util to grab HandleValue.

[josephlr]

One idea worth considering here is which direction we want to support conversion for the TPM types. We can support:

1. Using the new TPM type (transport.TPM) with the old TPM interface (io.ReadWriter/io.ReadWriteCloser) and library.
2. Using the old TPM type (io.ReadWriter) with the new TPM interface (transport.TPM) and library

This PR implements (2), but I'm wondering if we should implement (1) instead.

[matt-tsai]

One idea worth considering here is which direction we want to support conversion for the TPM types. We can support:

1. Using the new TPM type (transport.TPM) with the old TPM interface (io.ReadWriter/io.ReadWriteCloser) and library.
2. Using the old TPM type (io.ReadWriter) with the new TPM interface (transport.TPM) and library

This PR implements (2), but I'm wondering if we should implement (1) instead.

This PR implements 2 because the old tests still need RWC and the Direct Migration needs a RW in order to be made a transport tpm used for Execute. Correct me if I'm wrong but eventually we will only have 1 interface.
I think the option (option 2 in this case) where we change RW into transport TPMs might be wiser as it would require less changes to direct and would also be backwards compatible.

[josephlr]

@matt-tsai let us know when this is ready for review again (and the TODO comments are cleaned up).

[chrisfenner]

Thanks for the change, looks great. Only teeny nits

[josephlr]

Other than the Read implementation, LGTM

[josephlr]

Nice work on this!!!