Remove duplicate index from block_device_mapping table

Migration 186 (doesn't exist anymore) deleted the column virtual_name
from the the block_device_mapping table. This ended up modifying an
index (block_device_mapping_instance_uuid_virtual_name_device_name_idx)
to be identical to (block_device_mapping_instance_uuid_device_name_idx)

Closes-Bug: 1265839
Change-Id: Ib293220eb764e9810212d121ce7889e6aefc266e
Merge "Remove duplicate index from block_device_mapping table"