fix qemu-img error when 2nd time in OVF.
fix qemu-img error when 2nd time in OVF.

Signed-off-by: Hiroshi Miura <ff6fb3ed059786151e736af13627c08d24e9f922@linux.com>

adrahon/vagrant-kvm,rainforestapp/vagrant-kvm,adrahon/vagrant-kvm