libvirt: check value of need_legacy_block_device_info

Commit f9f247ef42c690bf1bd1ae65135c6e7fbd0d3c8c added support for this
value but did not actually check the value of the property set by
the driver.

Change-Id: Ifd9a8981b5c94d764166d782c79e0ae5a0ab045a