virtio-serial-bus: let chardev know the exact number of bytes requested

virtio-serial-bus: let chardev know the exact number of bytes requested

Using the virtqueue_avail_bytes() function had an unnecessarily
crippling effect on the number of bytes needed by the guest as reported
to the chardev layer in the can_read() callback.

Using the new virtqueue_get_avail_bytes() function will let us advertise
the exact number of bytes we can send to the guest.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>