Adds common image utils

This is the first step to merge the code in nova/virt/images.py
and cinder/image/image_utils.py.

This patch pulls up the QemuImgInfo class which is common
in the two modules above.
The test script is based on nova/tests/virt/test_images.py with
an additional test_qemu_info_blank testcase.

Partially implements bp imageutils

Change-Id: Ia07739492776180e579b03920aa32b9215ce941d
Merge "Adds common image utils"