Fix igxbe SRIOV VF (if_ixv) initialization bugs. The MAC address for
an if_ixv instance can now set at creation time, and the receive ring
tail pointer is correctly initialized (previously, things still worked
because the receive ring tail pointer was being fixed up as a side
effect of other activity).

Differential Revision: https://reviews.freebsd.org/D2922
Reviewed by: erj, gnn
Approved by: jmallett (mentor)
Sponsored by: Norse Corp, Inc.