This adds package arch inspection for deb-based systems. The patch
returns package arch as given by dpkg, which means that it returns
'amd64' instead of 'x86_64', for example. Whether or not we should
standardize this to match rpm output is up for debate. I see other
places such as guestfs_file_architecture that use the rpm-style arch.
I can understand though where a user of guestfs inspecting deb-based
hosts would rather get back the "native" strings.
Thoughts?