Add VirtualBox support to linuxkit run

You can use `linuxkit run vbox ...` to run using Oracle VirtualBox.
ISO or raw should be supported, and you can specify EFI.

Some more options may be useful in future.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

docs/platform-vbox.md

@@ -0,0 +1,30 @@
+# LinuxKit with VirtualBox
+
+LinuxKit can run using Oracle VirtualBox. This should work on OSX, Linux
+and Windows. The standard install should be sufficient.
+
+NB: Windows support is not currently working but should be fixed soon.
+
+## Boot
+
+The Virtualbox backend currently supports booting from disks or ISOs.
+It should work with either BIOS (default) or EFI
+(with `linuxkit run vbox --uefi ...`).
+
+## Console
+
+With `linuxkit run vbox` the serial console is redirected to
+stdio, providing interactive access to the VM.
+
+
+## Disks
+
+The Virtualbox backend support configuring a persistent disk using the
+standard `linuxkit` `-disk` syntax.  Multiple disks are
+supported and can be created in `raw` format; other formats that VirtualBox
+supports can be attached
+
+## Networking
+
+You can select the networking mode, which defaults to the standard `nat`, but
+some networking modes may require additional configuration.