spapr: consolidate the VCPU id numbering logic in a single place
    
    Several places in the code need to calculate a VCPU id:
    
        (cpu_index / smp_threads) * spapr->vsmt + cpu_index % smp_threads
        (core_id / smp_threads) * spapr->vsmt (1 user)
        index * spapr->vsmt (2 users)
    
    or guess that the VCPU id of a given VCPU is the first thread of a virtual
    core:
    
        index % spapr->vsmt != 0
    
    Even if the numbering logic isn't that complex, it is rather fragile to
    have these assumptions open-coded in several places. FWIW this was
    proved with recent issues related to VSMT.
    
    This patch moves the VCPU id formula to a single function to be called
    everywhere the code needs to compute one. It also adds an helper to
    guess if a VCPU is the first thread of a VCORE.
    
    Signed-off-by: Greg Kurz <groug@kaod.org>
    [dwg: Rename spapr_is_vcore() to spapr_is_thread0_in_vcore() for clarity]
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit 5d0fb1508e2d279da74ef4a103e8def9b52c6304)
    Signed-off-by: Greg Kurz <groug@kaod.org>