qemu: Actually clear bandwidth settings

qemu: Actually clear bandwidth settings

The virDomainSetInterfaceParameters implementation in qemu over
VIR_DOMAIN_AFFECT_CONFIG doesn't work as expected. When trying to
clear out the bandwidth settings for an interface, it has no
actual effect:

    virsh # domiftune --config $domain $interface
    inbound.average: 100
    inbound.peak   : 0
    inbound.burst  : 0
    outbound.average: 10
    outbound.peak  : 0
    outbound.burst : 0

    virsh domiftune --config $domain $interface 0 0

    virsh # domiftune --config $domain $interface
    inbound.average: 100
    inbound.peak   : 0
    inbound.burst  : 0
    outbound.average: 10
    outbound.peak  : 0
    outbound.burst : 0

But according to virsh man page:

    To clear inbound or outbound settings, use --inbound or
    --outbound respectfully with average value of zero.

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