conf: Move virDomainClearNetBandwidth() to src/hypervisor/

conf: Move virDomainClearNetBandwidth() to src/hypervisor/

The reason virDomainClearNetBandwidth() exists in src/conf/ is
that at the time its introduction we did not have a better place.
But now we do. Firstly, virDomainClearNetBandwidth() is
hypervisor agnostic code, but really has nothing to do with
domain configuration (it doesn't parse/format XML). Secondly, in
near future it'll call another function from src/hypervisor/ and
that's not really allowed from src/conf/.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>