Parametrize headless option for VirtualBox

If you need to start VirtualBox VMs without a GUI, for example, on a
remote computer, you can now set the 'headless' parameter in config.sh
to '1'.

DocImpact
Closes-Bug: #1304345

Change-Id: I868fa67335506a5e048d37411f5d5795b560a338

config.sh

@@ -178,3 +178,6 @@ fi
 vm_slave_first_disk_mb=65535
 vm_slave_second_disk_mb=65535
 vm_slave_third_disk_mb=65535
+
+# Set to 1 to run VirtualBox in headless mode
+headless=0

functions/vm.sh

@@ -159,8 +159,11 @@ start_vm() {
     name=$1
 
     # Just start it
-    #VBoxManage startvm $name --type headless
-    VBoxManage startvm $name
+    if [[ $headless == 1 ]]; then
+        VBoxManage startvm $name --type headless
+    else
+        VBoxManage startvm $name
+    fi
 }
 
 mount_iso_to_vm() {