virtio: console: Use better variable names for fill_queue operation

We want to keep track of the number of buffers added to a vq. Use
nr_added_bufs instead of 'ret'.

Also, the users of fill_queue() overloaded a local 'err' variable to
check the numbers of buffers allocated. Use nr_added_bufs instead of
err.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reported-by: Juan Quintela <quintela@redhat.com>