spice-qemu-char: Fix flow control in client -> guest direction

spice-qemu-char: Fix flow control in client -> guest direction

In the old spice-vmc device we used to have:
last_out = virtio_serial_write(&svc->port, p, MIN(len, VMC_MAX_HOST_WRITE));
if (last_out > 0)
   ...

Now in the chardev backend we have:
last_out = MIN(len, VMC_MAX_HOST_WRITE);
qemu_chr_read(scd->chr, p, last_out);
if (last_out > 0) {
   ...

Which causes us to no longer detect if the virtio port is not ready
to receive data from us. chardev actually has a mechanism to detect this,
but it requires a separate call to qemu_chr_can_read, before calling
qemu_chr_read (which return void).

This patch uses qemu_chr_can_read to fix the flow control from client to
guest.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>