Replace 'distrofamily' with feature tags

It turns out that the distribution hierarchy is not as reliable concept as you
might think. This patch removes distrofamily again.

Instead of distrofamily, we will add feature tags. This patch adds 2 feature
tags for Linux distributions:

package_format (eg rpm/dpkg)
package_management (eg rhn/yum/apt)

This change is reflected in the output of virt-inspector