virtio-net: use aliases instead of duplicate qdev properties
virtio-net-pci, virtio-net-s390, and virtio-net-ccw all duplicate the
qdev properties of their VirtIONet child. This approach does not work
well with string or pointer properties since we must be careful about
leaking or double-freeing them.
Use the QOM alias property to forward property accesses to the
VirtIONet child. This way no duplication is necessary.