pcnet: Add link state support

Update lnkst on link state changes so that guests can obtain this
information via reading back the LED output pin. Works for Linux but
not for guests that depend on the missing PHY.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>