XenAPINFS: Create volume from image (generic)

Fixes bug 1130706

Related to blueprint xenapinfs-glance-integration

This patch enables users to use any images recognised by qemu-img to
create volumes on XenAPINFS.

Change-Id: I9dd8ec237309da82ec7f73db1acb48e5b7411c9e
temporary commit

Change-Id: Id32f724b650b29c42156deb55e1666d4f44c3a3c
temporary commit

Change-Id: Ic7c4ecb81ce12270ebef5310a7970d4b8b64b6d6
Added XenAPI operations

Change-Id: Ie34fda88b2534ad43db16389d18efa98e4e0649e
Added volume_attached_here

Change-Id: I68bbe2047b896dcb56c7bea68044920f82c05a0c
Raise exception if non-xenserver image is used

Change-Id: Ic112e4e67877cdf0748c8b3e6a5c0ff515df0246
Add generic case for downloading an image

Change-Id: I648491590244b09083b018dcc724a6f687135541
fix typo

Change-Id: I575e57972b9b77fb587847c531dea7dbfe65528f
cover with tests

Change-Id: Ic0162bc1b4dc221a0d47192eacb96345516b6058