xen-netfront: add multiqueue support

Add support for multiple TX and RX queue pairs. The default number of queues
is set to 4, but can be easily changed from the sysctl node hw.xn.num_queues.

Also heavily refactor netfront driver: break out a bunch of helper
functions and different structures. Use threads to handle TX and RX.
Remove some dead code and fix quite a few bugs as I go along.

Submitted by: Wei Liu <wei.liu2@citrix.com>
Reviewed by: royger
Sponsored by: Citrix Systems R&D
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D4193