spapr: rename spapr_vcpu_id() to spapr_get_vcpu_id()
    
    The spapr_vcpu_id() function is an accessor actually. Let's rename it
    for symmetry with the recently added spapr_set_vcpu_id() helper.
    
    The motivation behind this is that a later patch will consolidate
    the VCPU id formula in a function and spapr_vcpu_id looks like an
    appropriate name.
    
    Signed-off-by: Greg Kurz <groug@kaod.org>
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit 14bb4486c819ea797a151b3e0fe53d6f5c7b3fc5)
    Signed-off-by: Greg Kurz <groug@kaod.org>