image_utils: misleading log message

The image_utils.get_qemu_data() function makes a call out to qemu-img in a try block, and if a ProcessExecutionError is raised, a message is logged saying that qemu-img is not installed:
https://opendev.org/openstack/cinder/src/commit/a92aa06e463f6322e8abdc2f570a801ea82dd1cb/cinder/image/image_utils.py#L655

During testing for Bug #1996188, Luigi noticed that if you are using a VMDK with a named extent where the named file isn't available, the call to qemu-img will return an error code that gets propagated up to image_utils as a ProcessExecutionError. You can make this happen by generating a VMDK like this:

$ qemu-img create -f vmdk mono.vmdk 1M -o subformat=monolithicFlat

This will create 2 files: mono.vmdk and mono-flat.vmdk. (mono.vmdk is the VMDK image; mono-flat.vmdk is the named extent holding the data.)

If you put mono.vmdk in glance, and then create a volume that contains the image, the volume will go to 'error' status, and you'll see something like this in the log:

2023-02-15 15:42:32.242 66 ERROR oslo_messaging.rpc.server ImageUnacceptable: Image 9f9449d8-c56a-4110-b0c0-30dc1f37fb67 is unacceptable: qemu-img is not installed and image is of type False. Only RAW images can be used if qemu-img is not installed.

We actually don't know that qemu-img is not installed. This is what happens when you ask qemu-img to give you info about the image (when mono-flat.vmdk is not present, which it isn't in glance):

$ qemu-img info --output=json mono.vmdk
qemu-img: Could not open 'mono.vmdk': Could not open 'mono-flat.vmdk': No such file or directory

$ echo $?
1

Since the call returns an error code, processutils will raise a ProcessExecutionError, even though qemu-img is in fact present.