Unit test case for more than 1 ephemeral disks in BDM

The following patch modifies the test case for
_get_instance_block_device_info to handle more than 1 ephemeral
disks in the BDM.  This is to address one of the comments made
in https://review.openstack.org/#/c/90583/ after it was already
approved for merger.  This patch also corrects a comment in
_get_instance_block_device_info to be more accurate.

Change-Id: I11e97f2e2ef999fa08af1693483d56bf8ad0e556
Closes-Bug: #1317321
Merge "Unit test case for more than 1 ephemeral disks in BDM"