Create a smaller disk image for the hwtest tree

Since the hwtest tree doesn't include the desktop and is considerably
smaller than the other trees, and since one of the uses of the tree
is writing to a USB disk for installation, build only a 4GB image
for the hwtest tree.

https://bugzilla.gnome.org/show_bug.cgi?id=737185
libqa: don't refer to the current/ symlink

ostree no longer creates a symlink pointing to the current deployment;
use libostree to find out the current deployment instead.

https://bugzilla.gnome.org/show_bug.cgi?id=737184