virtio-serial: Remove redundant check for 0-sized write request

The check for a 0-sized write request to a guest port is not necessary;
the while loop below won't be executed in this case and all will be
fine.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
