vmxnet3: Pad short frames to minimum size (60 bytes)

When running VMware ESXi under qemu-kvm the guest discards frames
that are too short. Short ARP Requests will be dropped, this prevents
guests on the same bridge as VMware ESXi from communicating. This patch
simply adds the padding on the network device itself.

Signed-off-by: Ben Draper <ben@xrsa.net>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
