Add boot_device support for vbox

In the Ironic ssh driver,
command set of 'set_boot_device' and 'get_boot_device' were not implemented
on VirualBox. This fixes it.

Change-Id: Icd19c7d1b5c11109cb6b400e99a6e7f2baa73d22
Closes-Bug: #1360017
Merge "Add boot_device support for vbox"