tpm: tpm_emulator: get and set buffer size of device

Convert the tpm_emulator backend to get the current buffer size
of the external device and set it to the buffer size that the
frontend (TIS) requests.

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