KVM: PPC: Book3S HV: Fix register usage when loading/saving VRSAVE

Commit 595e4f7e

 ("KVM: PPC: Book3S HV: Use load/store_fp_state
functions in HV guest entry/exit") changed the register usage in
kvmppc_save_fp() and kvmppc_load_fp() but omitted changing the
instructions that load and save VRSAVE.  The result is that the
VRSAVE value was loaded from a constant address, and saved to a
location past the end of the vcpu struct, causing host kernel
memory corruption and various kinds of host kernel crashes.

This fixes the problem by using register r31, which contains the
vcpu pointer, instead of r3 and r4.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>