vmxnet3: initialize buf array to prevent assertion error on reboot

Seems like on cold boot _buf is all zero, but after reboot it doesn't.
So we should initialize them.

fixed #484

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>