qdev: Drop hotplug check from bus_add_child()

Check is too restrictive and does not allow
to add children to just created bus during hotplug
when the bus is part of composite device.

Removing check from bus_add_child() doesn't affect
devices creatable with device_add/del commands since
they have a similar builtin check and patch will
allow to create complex composite devices during
hotplug.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
