tpm_emulator: Add a caching layer for the TPM Established flag

Add a caching layer for the TPM established flag so that we don't
need to go to the emulator every time the flag is read by accessing
the REG_ACCESS register.

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