XenAPINFS: Fix Volume always uploaded as vhd/ovf

Fixes bug 1131291

copy_volume_to_image was always using the glance plugin to do the
upload, and was ignoring the users request. Thus the image was always
uploaded as vhd/ovf. This patch is using the image utils to fix the
generic case.

Change-Id: If12c3d175b3d4437305ac4d2f954a2a42ee3a3c1
Merge "XenAPINFS: Fix Volume always uploaded as vhd/ovf"