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>
