test/py: Rework test_eficonfig to not use virt-make-fs

test/py: Rework test_eficonfig to not use virt-make-fs

The problem with using "virt-make-fs" to make a filesystem image is that
it is extremely slow. Switch to using the fs_helper functions we have
instead from the filesystem tests as these can add files to images and
are significantly faster and still do not require root access.

As this test already had a number of internal functions, add a
prepare_image function to do this part of the test.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>