CA-80935: Default VIF's MTU to network's MTU
CA-80935: Default VIF's MTU to network's MTU

Previously, a VIF would use its own MTU, if set, or default to 1500.  This was
not intended behaviour.

djs55/squeezed,sharady/xcp-networkd,koushikcgit/xcp-rrdd,simonjbeaumont/xcp-rrdd,koushikcgit/xcp-networkd,simonjbeaumont/xcp-rrdd,robhoes/squeezed,koushikcgit/xcp-rrdd,djs55/xcp-rrdd,koushikcgit/xcp-rrdd,djs55/xcp-networkd,koushikcgit/xcp-networkd,sharady/xcp-networkd,johnelse/xcp-rrdd,djs55/xcp-rrdd,johnelse/xcp-rrdd,djs55/xcp-networkd