launch: direct: Handle iface=virtio using virtio-blk path.

iface=virtio means use (old) virtio-blk.  Since there's an existing
path for using virtio-blk, use that instead of adding if=virtio (which
will fail on ARM).
arm: Error if iface=ide and skip tests that use iface=ide.

ARM doesn't support IDE.  (Well, that's not entirely true as I found a
reference to one extremely obscure ARM board that had IDE disks, but
qemu-system-arm doesn't appear to support them).