qmp: permit query-hotpluggable-cpus in preconfig state

qmp: permit query-hotpluggable-cpus in preconfig state

it will allow mgmt to query possible CPUs, which depends on
used machine(version)/-smp options, without restarting
QEMU and use results to configure numa mapping or adding
CPUs with device_add* later.

PS:
*) device_add is not allowed to run at preconfig in this series
   but later it could be dealt with by injecting -device
   in preconfig state and letting existing -device handling
   to actually plug devices

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1525423069-61903-10-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>