Revert "network: Check for QOS before blindly using it"

This reverts commit f4db846c32c0a1e99a0f62b340273e48f8a98ed3.

This patch results in the following error when trying to start
essentially any VM with default network:

unsupported configuration: QOS must be defined for network 'default'

Coverity didn't see that the bandwidth == NULL it complained about in
virNetDevBandwidthPlug was already checked properly in
networkCheckBandwidth, thus causing networkPlugBandwidth to return 0
and finish before a call to virNetDevBandwidthPlug would have been even
made.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>