retrive ovf disk interface and import to kvm box

This patch add function to retrieve disk interface from ovf
and respect it in generating kvm box.

It can be override in vagrant-kvm provider with config.
Some box such as arch linux, depends disk interface because of some boot loader(syslinux) need specific interface that is installed. 

Signed-off-by: Hiroshi Miura <miurahr@linux.com>