e1000: Pad short frames to minimum size (60 bytes)

The OpenIndiana (Solaris) e1000g driver drops frames that are too long
or too short.  It expects to receive frames of at least the Ethernet
minimum size.  ARP requests in particular are small and will be dropped
if they are not padded appropriately, preventing a Solaris VM from
becoming visible on the network.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
