util: vircgroupv2: add support for BFQ files

util: vircgroupv2: add support for BFQ files

In kernel 4.12 there was introduced new BFQ scheduler and in kernel
5.0 the old CFQ scheduler was removed.  This has an implication on
the cgroups file names.

If the CFQ controller is enabled we use one file:

    io.weight

The new BFQ controller expose one file with different name:

    io.bfq.weight

Except for different name they have different syntax.

io.weight:

    default $val
    major:minor $val

io.bfq.weight:

    $val

The difference is that BFQ doesn't support per-device weight.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>