qemu: capabilities: Extract availability of new cpu hotplug for machine types
    
    QEMU reports whether 'query-hotpluggable-cpus' is supported for a given
    machine type. Extract and cache the information using the capability
    cache.
    
    When copying the capabilities for a new start of qemu, mask out the
    presence of QEMU_CAPS_QUERY_HOTPLUGGABLE_CPUS if the machine type
    doesn't support hotpluggable cpus.