test_api_base: use image name in _copy_image

test_api_base: use image name in _copy_image

This is a minor clean-up which changes the '_copy_image()' so that it expects
the image to be a path and does not allow for file-like objects. This is
cleaner than seeking the file-like object in this function, which changes the
object state.

This change is rather mechanical, and this is actually a preparation for the
upcoming changes, where I am going to make '_copy_image()' and some other
function generic, and then use them in a new test which I am going to add.

Change-Id: I81e7e2d15676794f9d5e0e93ce62e0945b95a6d0
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>