Make the e1000 the default network adapter for the pc target.

Make the e1000 the default network adapter for the pc target.

The ne2k is an ancient card that performs pretty terribly under QEMU.  In many
modern OSes, there is no longer drivers available for the ne2k.

Switch the default network adapter to e1000.  This card is more widely
suppported and performs rather well under QEMU.  There may be very old OSes
that had a ne2k driver but not an e1000 driver but I think this is likely the
exception.

I think the average user is better served with an e1000 vs ne2k.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>