tests: usb: usb-storage hotplug test

usb-storage is different from usual usb devices
in that it uses a child SCSI bus for underlying storage.
This commit verifies that the SCSI bus is hotpluggable, as
hotplug operation wouldn't succeed without it.

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