virnetdevbandwidth.c: Separate tc filter creation to a function

virnetdevbandwidth.c: Separate tc filter creation to a function

Not only this simplifies the code a bit, it prepares the
environment for upcoming patches. The new
virNetDevBandwidthManipulateFilter() function is capable of both
removing a filter and adding a new one. At the same time! Yeah,
this is not currently used anywhere but look at the next commit
where you'll see it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>