Sorry for my misleading previous answer, I thought you are asking how to
create pre/thin allocated disk. As for the format, you cannot choose it. As
for the defaults, Benny already answered this.
I also have block storage on my environment. How do I observe the
type of a
vm image (LV)?
What do you exactly mean? Do you ask about image format (raw vs. cow)? If so,
you can deduce it from Benny's answer, based on the facts on which domain is
disk stored and if it's pre-allocated or not. If you are asking how to check
it on the storage, you can check disk metadata or you can use directly qemu-
img utility (qemu-img info /path/to/the/disk)