[Libguestfs] A few easy dpkg optimizations for supermin

I have done some printf profiling and found that supermin's calls to dpkg for individual packages are quite expensive. Here are some patches that gather all information on demand where possible. On my Debian/unstable-based workstation at home, preparing a minimal appliance using $ ./supermin --use-installed --prepare bash -o supermin.d now takes ~3.5s (previously ~15s). Turning that appliance into an image using $ ./supermin --build -f ext2 supermin.d -o appliance.d now takes ~6s (previously ~14.5s).