qemu: Forbid config when topology based cpu count doesn't match the config
    
    As of qemu commit:
    commit a32ef3bfc12c8d0588f43f74dcc5280885bbdb30
    Author: Thomas Huth <thuth@redhat.com>
    Date:   Wed Jul 22 15:59:50 2015 +0200
    
        vl: Add another sanity check to smp_parse() function
    
    v2.4.0-952-ga32ef3b
    
    configuration where the maximum CPU count doesn't match the topology is
    rejected. Prior to that only configurations where the topology would
    contain more cpus than the maximum count would be rejected.
    
    Use QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS as a relevant recent enough
    witness to avoid breaking old configs.