network: Check for QOS before blindly using it

network: Check for QOS before blindly using it

If networkAllocatePort calls networkPlugBandwidth eventually the
port->bandwidth would be passed to virNetDevBandwidthPlug which
requires that the parameter is non-NULL.  Coverity additionally
notes that since (!port->bandwidth) is checked earlier in the
networkAllocatePort method that the subsequent call to blindly
use if for a function that requires it needs to check.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>