Add block_device_mapping_get_all_by_instance to virtapi

Add another conductor api method to the virtapi.  This is needed by the
following patch: https://review.openstack.org/#/c/42812

Part of blueprint qemu-assisted-snapshots

Change-Id: I21d0ebe9f83423d504f7626f0d048b31945156d9
Merge "Add block_device_mapping_get_all_by_instance to virtapi"