net: virtual: Clear multicast bit when generating link address

The least significant bit of the first octet of a MAC address is a
unicast/multicast bit. The bit should be cleared when generating a
random link address for a virtual interface. Any frames from such
interface/address will be dropped by the network as invalid if
the multicast bit is set.

Signed-off-by: Jack Chistyakov <jack.chistyakov@outlook.com>