tpm: extend TPM TIS with state migration support

Extend the TPM TIS interface with state migration support.

We need to synchronize with the backend thread to make sure that a command
being processed by the external TPM emulator has completed and its
response been received.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>