The TPM backend uses a GThreadPool to handle IO in a seperate
thread. However, GThreadPool isn't integrated with Qemu main loops,
making it unnecessarily complicated to deal with.

Qemu has a AIO threadpool, that is better integrated with loops and
various IO functions, provides completion BH by default etc.

Remove the only user of GThreadPool from qemu, use AIO threadpool.

Note that the backend:
- no longer accepts queing multiple requests (unneeded so far)
- increase ref to itself when handling a command, for extra safety
- tpm_backend_thread_end() is renamed tpm_backend_finish_sync() and
will wait for completion of BH (request_completed), which will help
migration handling.

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