Enable live block migration when using iSCSI volumes

DO NOT MERGE

Draft fix for bug 1160323.

Depends on internal XAPI commit 11de810d740cc08946e90752e86728a6e98574eb
with commit message CP-4498: Added relaxed check mode to allow
iSCSI SRs during XSM.

It also depends on a configuration change which must be documented and added to devstack:
* Set "relax-xsm-sr-check = true" in /etc/xapi.conf

This commit makes the following changes:
* Attach the SR on the destination host prior to migrate
* Returns the SR-ref from the pre_migrate call (API change)
* Populates the XS VDI map with the sr-ref on the destination host
* Removes the connection to the SR from the source host post-migrate
* Added plugin to determine if iSCSI block migration is enabled

Change-Id: I917d9cf094190d636f4b9e14f6c8e728ff85af0e
Fixes: bug 1160323
Added tests, addressed Mate's comments

Change-Id: I3e447e7e2b57a6d4adc777b8d8e9c227c1d8a138