conf: Introduce virNetDevBandwidthValidate()

conf: Introduce virNetDevBandwidthValidate()

This function validates whether parsed limits are within range as
defined by 'tc' sources (since we use tc to set QoS; or OVS which
then uses tc too). The 'tc' program stores speeds in 64bit
integers (unit is bytes per second) and sizes in uints (unit is
bytes). We use different units: kilobytes per second and
kibibytes and therefore we can parse values larger than 'tc' can
handle and thus need a function to check if values still fit.

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