networkStartNetworkVirtual: Don't overwrite error in 'err5'

networkStartNetworkVirtual: Don't overwrite error in 'err5'

If there's an error when setting up QoS on a bridge the control
jumps over to 'err5' label. Here, the virNetDevBandwidthClear()
is called to clear out any partially set QoS. This function can
also report an error which would overwrite the actual error that
caused us jumping here. Use virErrorPreserveLast() to preserve
the original error.

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