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>

spice-qemu-char.c

@@ -36,14 +36,13 @@ static int vmc_write(SpiceCharDeviceInstance *sin, const uint8_t *buf, int len)
 
     while (len > 0) {
         last_out = MIN(len, VMC_MAX_HOST_WRITE);
-        qemu_chr_read(scd->chr, p, last_out);
-        if (last_out > 0) {
-            out += last_out;
-            len -= last_out;
-            p += last_out;
-        } else {
+        if (qemu_chr_can_read(scd->chr) < last_out) {
             break;
         }
+        qemu_chr_read(scd->chr, p, last_out);
+        out += last_out;
+        len -= last_out;
+        p += last_out;
     }
 
     dprintf(scd, 3, "%s: %lu/%zd\n", __func__, out, len + out);