add virtio-scsi driver

virtio-scsi is a simple HBA that talks to the host via a single
vring.  The implementation looks like a hybrid of usb-msc and
virtio-blk.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/Makefile b/Makefile
index 00da02c..abbac8c 100644
--- a/Makefile
+++ b/Makefile
@@ -15,7 +15,7 @@
     kbd.c pci.c serial.c clock.c pic.c cdrom.c ps2port.c smp.c resume.c \
     pnpbios.c pirtable.c vgahooks.c ramdisk.c pcibios.c blockcmd.c \
     usb.c usb-uhci.c usb-ohci.c usb-ehci.c usb-hid.c usb-msc.c \
-    virtio-ring.c virtio-pci.c virtio-blk.c apm.c ahci.c
+    virtio-ring.c virtio-pci.c virtio-blk.c virtio-scsi.c apm.c ahci.c
 SRC16=$(SRCBOTH) system.c disk.c font.c
 SRC32FLAT=$(SRCBOTH) post.c shadow.c memmap.c coreboot.c boot.c \
     acpi.c smm.c mptable.c smbios.c pciinit.c optionroms.c mtrr.c \