VirtualBox driver should replace only last occurence of suggested VM name

When suggested VM name is present also in the path, the bug encountered:

<pre>
INFO subprocess: Starting process: ["/usr/bin/VBoxManage", "import", "/media/arch/srs-vagrant/.vagrant.d/boxes/debian740_provisioned/virtualbox/box.ovf", "--vsys", "0", "--vmname", "srs-vagrant_1403592072958_54939", "--vsys", "0", "--unit", "10", "--disk", "/media/arch/<b>srs-vagrant_1403592072958_54939/vbox/srs-vagrant</b>/box-disk1.vmdk"]
</pre>

But should be

<pre>
INFO subprocess: Starting process: ["/usr/bin/VBoxManage", "import", "/media/arch/srs-vagrant/.vagrant.d/boxes/debian740_provisioned/virtualbox/box.ovf", "--vsys", "0", "--vmname", "srs-vagrant_1403600101329_78183", "--vsys", "0", "--unit", "10", "--disk", "/media/arch/<b>srs-vagrant/vbox/srs-vagrant_1403600101329_78183</b>/box-disk1.vmdk"]
</pre>