linuxbridge-agent: make vxlan unicast check more efficent

A list with the full range of vxlan VNIs (1 to 2**24) was using
over 300MB of memory. This patch uses xrange instead of range.

Closes-Bug: #1393362
Change-Id: I21ccab758e7911712690fd5e732f64361e809264
(cherry picked from commit 0bf69ad80fb77773e767e116357cf6e1666b3f5d)
Merge "linuxbridge-agent: make vxlan unicast check more efficent" into stable/juno