My Intel Server board used to deliver a whopping 3MB/s of sata throughput on all ports with Oracle VM, after a lot of poking around I figured the problem was the disks were not in AHCI mode.
Build a new initrd (because otherwise it won’t even find your rootdev…):
mv /boot/initrd-`uname -r`.img /boot/initrd-`uname -r`.backup
mkinitrd –allow-missing –preload=ahci –force-scsi-probe /boot/initrd-`uname -r`.img `uname -r`
enter bios and go to advanced -> sata -> sata mode and change that to AHCI.
boot up and enjoy proper 120+ data rates from there on.