Change the SCSI controller type when editing a harddisk only when creating a VM.

Change the SCSI controller type when editing a harddisk only when creating a VM.

If not, the following could happen:
 * user has a VM with two disks attached to a LSI controller
 * adding a third disk, he notices the iothread option, and clicks it
 * the scsihw type for all disks is changed now to virtio-scsi-single
 * on next cold restart, STOP 0x0000007B INACCESSABLE_BOOT_DEVICE for a Windows VM

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>