hw/net: e1000: Correct the initial value of VET register

hw/net: e1000: Correct the initial value of VET register

The initial value of VLAN Ether Type (VET) register is 0x8100, as per
the manual and real hardware.

While Linux e1000 driver always writes VET register to 0x8100, it is
not always the case for everyone. Drivers relying on the reset value
of VET won't be able to transmit and receive VLAN frames in QEMU.

Reported-by: Markus Carlstedt <markus.carlstedt@windriver.com>
Signed-off-by: Christina Wang <christina.wang@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>