cinder-volume: install qemu-img for conversions

Updates the cinder-volume element so that it installs the
qemu-img package. This is required in order to support
boot from volume conversions from Glance images (qcow2).

Change-Id: I1c6fb4692166b173092395967ffe9fe88ffdb602
Partial-bug: #1340479
Merge "cinder-volume: install qemu-img for conversions"