This patch refactors logic for creating a temporary mount point,
mounting an image, running arbitrary code on it, unmounting the image
and removing the mount point. It immediately uses it in the buildinstall
phase.
Similar mounting is present in product_img phase as well, but due to
different usage pattern it's not changed yet.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>