block: qemu-iotests, add quotes to $TEST_IMG usage in 061

block: qemu-iotests, add quotes to $TEST_IMG usage in 061

When creating images with backing files in the test, the backing
file argument was not quoted properly.  This caused the test to fail
when run from a pathname with a space.  Pass the backing argument in
with the -b option to _make_test_img, so it can be properly quoted.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>