Libvirt: fix KeyError in set_vif_bandwidth_config

The set_vif_bandwidth_config function must be able to get a parameter that has
no 'extra_specs' field and not fail.

This fixes bug 1214363.

Change-Id: I576fb087c7bcda9f6662c4df5b480668b1f23454
Merge "Libvirt: fix KeyError in set_vif_bandwidth_config"