Enables packetfilter ext in NEC plugin based on its driver config
Enables packetfilter ext in NEC plugin based on its driver config

packet filter extension in NEC plugin should be determined based on
a return value of filter_supported method in the backend driver.
Previously filter_supported was not called actually and packetfilter
extension was always enabled.

Fixes bug #1133100

Change-Id: I1334c7695e64bc942c0ecce2bd85e5a92e75b388

gkotton/vmware-nsx,gkotton/vmware-nsx