qemu: Refresh memory size only on fresh starts
    
    Qemu unfortunately doesn't update internal state right after migration
    and so the actual balloon size as returned by 'query-balloon' are
    invalid for a while after the CPUs are started after migration. If we'd
    refresh our internal state at this point we would report invalid current
    memory size until the next balloon event would arrive.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1242940