qemu-iotests: only run 071 on qcow2

qemu-iotests: only run 071 on qcow2

The 071 test is designed for IMGFMT=qcow2 because it uses the l2_load
blkdebug event.  Its output filtering also assumes that IMGFMT is not
raw since 071.out contains "format=raw" but IMGFMT=raw would filter the
output to "format=IMGFMT".

Perhaps the test case can be rewritten to be more generic, but for now
let's document that it was only supposed to work with qcow2.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>