vmxnet3: add command to set ring buffer sizes

This patch adds a new command to set ring buffer sizes. This is
required to pass the buffer size information to passthrough devices.
For performance reasons, with version7 and later, ring1 will contain
only mtu size buffers (bound to 3K). Packets > 3K will use both ring1
and ring2.

Also, ring sizes are round down to power of 2 and ring2 default
size is increased to 512.

Signed-off-by: Ronak Doshi <doshir@vmware.com>
Acked-by: Guolin Yang <gyang@vmware.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>