vTPM: get the buffer allocated for event log instead of the actual log

The OS should ask Power Firmware (PFW) for the size of the buffer
allocated for the event log, instead of the size of the actual
event log.  It then passes the buffer adddress and size to PFW in
the handover process, into which PFW copies the log.

Signed-off-by: Hon Ching(Vicky) Lo <honclo@linux.vnet.ibm.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>