virtio-pci: Drop BusState::allow_hotplug

virtio-pci: Drop BusState::allow_hotplug

virtio-pci-bus is an internal object of composite
virtio-pci device and it doesn't participate in
-device/device_add hotplug flow, and since it's
not required by bus_add_child() that BUS must
be hotpluggable to be able to add child at runtime,
it's possible to drop not needed 'allow_hotplug'
field.

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