LIO target needs better detection for broken targets

[Eric Harney]

There are cases where ensure_export() in LIO does not do the right thing, particularly after a system reboot where the LVM VG and LVs may not be in the expected state, but the LIO targets have been restored.

ensure_export() does a self._get_targets() and does not detect if the target "exists" in LIO/targetcli but does not have a block backstore attached to it.

Something in this path needs to detect that the target is missing its backstore and recreate it.