pseries: Small cleanup to H_CEDE implementation

The H_CEDE hypercall implementation for the pseries machine doesn't trigger
quite the right path in the main cpu exec loop.  We should set exit_request
to pop up one extra level and recheck state, and we should set the
exception_index to EXCP_HLT (H_CEDE is roughly equivalent to the hlt
instruction on x86).

In practice, this doesn't really matter except for KVM, and KVM implements
H_CEDE internally so we never hit this code path.  But we might as well
get it right, just in case it matters some day.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
