xen-netback: vif counters from int/long to u64

This patch fixes an issue where the type of counters in the queue(s)
and interface are not in sync (queue counters are int, interface
counters are long), causing incorrect reporting of tx/rx values
of the vif interface and unclear counter overflows.
This patch sets both counters to the u64 type.

Signed-off-by: Mart van Santen <mart@greenhost.nl>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>