net/vmxnet3: support stats reset

net/vmxnet3: support stats reset

'stats_reset()' callback was missing because the device backend doesn't
support it.

This commit adds a workaround to this and implements the callback by
taking a snapshot of the stats (SNAPSHOT) each time 'stats_reset()'
is called.  When getting stats with 'stats_get()', hw stats which
always increase reduce SNAPSHOT stats.
That's how we get the "real" stats since the last 'stats_reset()'.

Signed-off-by: Yogev Chaimovich <yogev@cgstowernetworks.com>
Acked-by: Yong Wang <yongwang@vmware.com>