tests: usb: usb-uas hotplug test

checks that it's possible to hotplug usb-uas HBA and
then if it's possible to hot(un)plug scsi-disk to it.
Thest basically covers hot(un)plug on dummy HBAs
without means of hot(un)plug notification of the guest.

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