Add Compute API validations for block device map

This patch adds validations to return 400 BadRequest to
Create server(s) with block device mappings where appropriate
and related unit tests

Fixes bug 1094244

Change-Id: I778d3ed4b0871561774594e950ce98b6a1ecfcb9
Merge "XenAPI: Fix volume detach"
Merge "Add Compute API validations for block device map"