KVM: PPC: BookE: Add GET/SET_ONE_REG interface for VRSAVE

This makes the VRSAVE register value for a vcpu accessible through
the GET/SET_ONE_REG interface on Book E systems (in addition to the
existing GET/SET_SREGS interface), for consistency with Book 3S.

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