tpm-passthrough: workaround a possible race

The TPM backend processing thread has common shared variable race
issues. (they should not be so easy to reach since guest interaction
with the device is slow compared to host emulation)

An obvious one is setting op_cancelled from device thread after
calling write(cancel_fd). The backend thread may return before the
device thread has set the variable. Instead set it before
cancellation. Even if the write() failed, the end result is command
get possibly cancelled (even if cancellation came from external
sources it doesn't matter much).

It's worth to consider removing the backend processing thread for now.

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>
