Enable live block migration when using iSCSI volumes

Fix for bug 1160323.

Depends on a version of XAPI supporting relax-xsm-sr-check.
Currently no release versions support this, so the change at
https://github.com/xen-org/xen-api/pull/1116 would need to be applied
to the source to enable this.

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

Associated tempest test at https://review.openstack.org/#/c/26478/

Change-Id: I917d9cf094190d636f4b9e14f6c8e728ff85af0e
Fixes: bug 1160323