powerpc/kvm: Fix "PR" KVM implementation of H_CEDE

H_CEDE should enable the vcpu's MSR:EE bit. It does on "HV" KVM (it's
burried in the assembly code though) and as far as I can tell, qemu
does it as well.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alexander Graf <agraf@suse.de>