system: Make CPU feature detection independent

Previously the CPU feature detection relied on libvirt. This lead to two
kind of problems:
1. libvirt is only providing names fo rthe closest known/baseline CPUs
2. the old logic failed in cases where libvirtd wasn't available

Change-Id: Ie321d633508239f70e4e59bcc94ba2e16a436449
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1096163
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1065831
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1062111
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>