issue #437: Only allow a bootable image file to be used on template

While creating a template from a image file, the latter must be a
bootable image.
As probe_image() does all the verifications regarding the image file,
the exception must be raised in order to inform user about the problem.
Also update the test case accordingly.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>