Fix backcompat for hotplug of SCSI controllers

Fix backcompat for hotplug of SCSI controllers

SCSI controllers have no trouble existing without any attached
disks. This could be achieved with the (legacy) monitor syntax

  pci_add pci_addr=auto storage if=scsi

This is now denied with

  scsi requires a backing file/device.
  failed to add if=scsi

There is no need for this denial and it breaks compatability
with existing QEMU usage, so remove the check for presence
of a drive.

  Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit ec7efac4a967c650ae1cd8cebe28e7c69cbe3864)