test: Use drive_add/device_add instead of pci_add to add disks

Recent qemu (around 1.6.1) no longer have the pci_add monitor
command.

So I've migrated this to use drive_add and device_add. We need to check that this works on Fedora 19 qemu as well.

In the process, I'm using a SCSI bus to hot plug the disks. I tried with virtio, but qemu told me that it wasn't supported. Odd, because it used to work with pci_add on earlier qemu versions. Anyway, using SCSI, is more realistic for hot plug testing.