Fix live-migration failure in FC multipath case

Currently, /dev/dm-<NUM> instead of /dev/mapper/<multipath_id> is
used to access multipath FC volumes by Compute Node and
multipath_id in connection_info is not maintained properly and
may be lost during connection refreshing.

This implementation will make source Compute Node and destination
Compute Node fail to disconnect/connect to volumes properly and
result in live-migration failure.

To fix it, /dev/mapper<multipath_id> will be used instead of
/dev/dm-<NUM> to access multipath devices, just like iSCSI multipath
implementation, and logic to preserve the unique (across Compute
Nodes) multipath_id is also added.

Change-Id: I17f15852c098af88afd270084c62eb87693c60d4
Closes-Bug: #1327497
Merge "Fix live-migration failure in FC multipath case"