iotests/024: exclude 'backing file format' field from the output

iotests/024: exclude 'backing file format' field from the output

Apparently 'qemu-img info' doesn't report the backing file format field
for qed (as it does for qcow2):

$ qemu-img create -f qed base.qed 1M && qemu-img create -f qed -b base.qed -F qed top.qed 1M
$ qemu-img create -f qcow2 base.qcow2 1M && qemu-img create -f qcow2 -b base.qcow2 -F qcow2 top.qcow2 1M
$ qemu-img info top.qed | grep 'backing file format'
$ qemu-img info top.qcow2 | grep 'backing file format'
backing file format: qcow2

This leads to the 024 test failure with -qed.  Let's just filter the
field out and exclude it from the output.

This is a fixup for the commit f93e65ee51 ("iotests/{024, 271}: add
testcases for qemu-img rebase").

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Message-ID: <20240730094701.790624-1-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>