QEMU code doesn't generally have assert() for mandatory
callbacks/function pointers, probably because the crash is pretty
obvious. Document the methods instead of going into the code.

Make get_tpm_options() mandatory to implement (since all
backend implementation have it).

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>