qemu: Clean up BuildRequires
    
    Drop outdated:
        nss-devel (old libcacard)
        rsync (no longer used)
        which (no longer used)
        pciutils-devel (no longer used)
    
    Add libcap-ng-devel for extra qemu-bridge-helper restrictions
    Document all BuildRequires
    Separate buildsystem bits vs feature bits