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: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>