spapr: add rtas_st_buffer_direct() helper

This is similar to the existing rtas_st_buffer(), but for case where
the guest is not expecting a length-encoded byte array. Namely,
for calls where an "work area" buffer is used to pass around
arbitrary fields/data.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
*spapr_rtas: fix configure_connector memory access