qapi: Report support for -device cpu hotplug in query-machines

qapi: Report support for -device cpu hotplug in query-machines

For management apps it's very useful to know whether the selected
machine type supports cpu hotplug via the new -device approach. Using
the presence of 'query-hotpluggable-cpus' alone is not enough as a
witness.

Add a property to 'MachineInfo' called 'hotpluggable-cpus' that will
report the presence of this feature.

Example of output:
    {
        "hotpluggable-cpus": false,
        "name": "mac99",
        "cpu-max": 1
    },
    {
        "hotpluggable-cpus": true,
        "name": "pseries-2.7",
        "is-default": true,
        "cpu-max": 255,
        "alias": "pseries"
    },

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>