spapr-rtas: add CPU argument to RTAS calls

RTAS is a hypervisor provided binary blob that a guest loads and
calls into to execute certain functions.  It's similar to the
vsyscall page in Linux or the short lived VMCI paravirt interface
from VMware.

The QEMU implementation of the RTAS blob is simply a passthrough
that proxies all RTAS calls to the hypervisor via an hypercall.

While we pass a CPU argument for hypercall handling in QEMU, we
don't pass it for RTAS calls.  Since some RTAs calls require
making hypercalls (normally RTAS is implemented as guest code) we
have nasty hacks to allow that.

Add a CPU argument to RTAS call handling so we can more easily
invoke hypercalls just as guest code would.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
