qemu-iotests: Split qcow2 only cases in 048

Format "raw" doesn't always work on certain file systems (e.g. tmpfs).
Use qcow2 to make the allocation status explicit and split into a new
case.

[Resolved merge conflict due to "qemu-io> " prompt filter, added 074 to
group file, and fixed up s/048/074/ copy-paste mistake.
--Stefan]

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>