target-ppc: force update of msr bits in cpu_post_load

Since env->msr has already been restored by the time cpu_post_load is called,
make sure that ppc_store_msr() is explicitly called with all msr bits except
MSR_TGPR marked as invalid.

This solves the issue where MSR flags aren't set correctly when restoring a VM
snapshot, in particular the internal env->excp_prefix value when MSR_EP has
been altered by a guest.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Alexander Graf <agraf@suse.de>
