xen-netfront: always set num queues if possible

xen-netfront: always set num queues if possible

If netfront connects with two (or more) queues and then reconnects with
only one queue it fails to delete or rewrite the multi-queue-num-queues
key and netback will try to use the wrong number of queues.

Always write the num-queues field if the backend has multi-queue support.

Signed-off-by: Chas Williams <3chas3@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 812494d9a0cacf77e0a538be18183c7b471812aa)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>