gve: Add consumed counts to ethtool stats

Being able to see how many descriptors are in-use is helpful
when diagnosing certain issues.

Signed-off-by: Jeroen de Borst <jeroendb@google.com>
Signed-off-by: Jordan Kim <jrkim@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>