Add db.block_device_mapping_get_by_id

Add a new db API method for getting a block device mapping for a given
volume ID.  This is needed when Cinder calls over to Nova asking nova to
complete a snapshot operation on its behalf.

Related to blueprint qemu-assisted-snapshots

Change-Id: If3f011e2db14235f4137c9d9cb610f0c4cd7954a
Merge "Add db.block_device_mapping_get_by_id"