qemu_storage: Error test if parent dir of image was not found

When running tests on an autotest grid, it is customary
that the installed KVM hosts have a directory structure
under /var/lib/virt_test already prepared, usually a mix
of local directories and NFS mounts. We've noticed that
some hosts (RHEL5) misbehave and sometimes the NFS dirs
are not available at the time the tests run.

So, when creating an image with qemu-img, if the parent
directory of the image file does not exist, instead of
working around the problem, fail the test altogether,
since this usually means a deeper setup issue that
must be investigated. The qemu-img, or dd command will
fail anyway, and when they do, it is hard to figure
out what exactly happened.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>