XenAPI: Calculate disk_available_least

The disk_available_least item is now needed for the tempest test.
Calculate based on the virtual-allocation from the SR.

Change-Id: I1a08f229de7cd77d4798472c82304247fae290e5
Closes-bug: 1300242
Add helper method to determine disk size from instance properties

Add a _disk_size_from_instance method which by using the disk name
determine the size of the disk refered in the instance properties.
Update the method finish_migration to use this new helper.

Change-Id: I45bd3804fdabff35189421d01a0503c886c24a4c
Related-Bug: #1298975
Xen: Retry plugin call after connection reset

When using the call_plugin_serialized_with_retry session method it will
retry if the plugin raises a RetryableError, but not if it loses the
connection to xenapi.  This adds a retry for the 'Connection reset by
peer' error that is occasionally seen.

Change-Id: Ibb36a0170e0e807ad7a4198df7b6e574c4279bed
Closes-Bug: #1200591
Merge "Xen: Retry plugin call after connection reset"
Merge "Add helper method to determine disk size from instance properties"
Merge "XenAPI: Calculate disk_available_least"