unattended_install: fix incorrect dst in copy nfs image

We should use get_image_filename function to get image file name to
avoid generate a incorrect image file name when use different image
backend.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1879 from xutian/fix_copy_image_nfs

unattended_install: fix incorrect dst in copy nfs image