Fix packet size checks and remove bad casts.

Fix packet size checks and remove bad casts.

Because dhcpsz was an uninitialized ssize_t, it was possible that a
garbage "packet" would be queued on the receiving end of the virtio
network device.

Change the type to size_t and add proper checks based on it being
greater than zero. Remove the cast of ssize_t to uint64_t that also
caused garbage sizes when dhcpsz was unintialized and set at runtime
to something < 0.