oslo.utils is planning to make JSON the default output format parsed
when creating QemuImgInfo objects. As such this change makes JSON the
default output_format requested when calling qemu-img info.
The majority of this change is actually test removal from
nova.tests.unit.virt.libvirt.test_utils as these human readable qemu-img
based tests now duplicate tests found in oslo.utils itself.
Change-Id: I56676713571e79f05ee3f0bffc5da8386e02c5d4