labs: enable host I/O cache

This patch enables host I/O cache for the VMs created by the labs
scripts.

Without host I/O cache, occasional VM filesystem corruption occured on
two out of three host systems when we started taking snapshots after
every service install on the controller VM. Typically, the affected
script would just hang. The issue is somewhat elusive: it occured only
on two out of three test hosts, and the longest observed streak of
correct controller VM builds in a row was 16 (number 17 ended with
filesystem corruption).

With host I/O cache enabled, 125 builds were successful and none failed.

Change-Id: Ib36f44fdbf9366ef781df82f225e5ac8c5bb678f

labs/lib/osbash/virtualbox.functions

@@ -378,6 +378,7 @@ function vm_create {
     else
         $VBM storagectl "$vm_name" --name SATA --add sata --portcount 1
     fi
+    $VBM storagectl "$vm_name" --name SATA --hostiocache on
 
     $VBM storagectl "$vm_name" --name IDE --add ide
     echo >&2 "Created VM \"$vm_name\""