libvirt: do not fail instance destroy, if mount_device is missing

In some cases, a volume does not have a mount_device field populated
in the device_mapping, especially for LXC instances. LXC instances
does not have a disk_bus mapping in get_dev_prefix_for_disk_bus()
and that results in mount_device being set to None.

Related to blueprint libvirt-start-lxc-from-block-devices

Change-Id: Idbd4ced0337c64003af08ac0497008878fb1c2e1
Merge "libvirt: do not fail instance destroy, if mount_device is missing"