KVM: PPC: Book3S HV: Save and restore CR in __kvmppc_vcore_entry

The ABI specifies that CR fields CR2--CR4 are nonvolatile across function
calls.  Currently __kvmppc_vcore_entry doesn't save and restore the CR,
leading to CR2--CR4 getting corrupted with guest values, possibly leading
to incorrect behaviour in its caller.  This adds instructions to save
and restore CR at the points where we save and restore the nonvolatile
GPRs.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>