tpm: reorganize headers and split hardware part

The TPM subsystem does not have a full front-end/back-end separation.
The sole available backend, tpm_passthrough, depends on the data
structures of the sole available frontend, tpm_tis.

However, we can at least try to split the user interface (tpm.c) from the
implementation (hw/tpm).  The patches makes tpm.c not include tpm_int.h,
which is shared between tpm_tis.c and tpm_passthrough.c; instead it
moves more stuff to tpm_backend.h.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>