xenapi: implement xenserver image to volume

xenapi: implement xenserver image to volume

Amend image_utils, so that it could handle xenserver type images (vhd
chains inside a tgz archive). This requires the vhd-util binary, if the
image contains a vhd chain (e.g.: snapshot)

DocImpact
- Copy a XenServer type OpenStack image (tgz -ed vhd chain) is now
  supported.
- To use the above functionality, the cinder box must have access to the
  vhd-util binary. In ubuntu, this is done by apt-get installing
  blktap-utils.

Change-Id: I99b58868439c4d5187c2fe3666110bfda0924181