virtio-serial-bus: keep port 0 reserved for virtconsole even on unplug

We keep port 0 reserved for compat with older guests, where only
virtio-console was expected.  Even if a system is started without a
virtio-console port, port #0 is kept aside.  However, after a
virtconsole port is unplugged, port id 0 became available, and the next
hotplug of a virtserialport caused failure due to it not being a console
port.

Steps to reproduce:

$ ./x86_64-softmmu/qemu-system-x86_64 -m 512 -cpu host -enable-kvm -device virtio-serial-pci -monitor stdio  -vnc :1
QEMU 2.0.91 monitor - type 'help' for more information
(qemu) device_add virtconsole,id=p1
(qemu) device_del p1
(qemu) device_add virtserialport,id=p1
Port number 0 on virtio-serial devices reserved for virtconsole devices for backward compatibility.
Device 'virtserialport' could not be initialized
(qemu) quit
Reported-by: Ndengmin <mdeng@redhat.com>
Reviewed-by: NAmos Kong <akong@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAmit Shah <amit.shah@redhat.com>

hw/char/virtio-serial-bus.c

@@ -797,10 +797,18 @@ static void add_port(VirtIOSerial *vser, uint32_t port_id)
 static void remove_port(VirtIOSerial *vser, uint32_t port_id)
 {
     VirtIOSerialPort *port;
-    unsigned int i;
 
-    i = port_id / 32;
-    vser->ports_map[i] &= ~(1U << (port_id % 32));
+    /*
+     * Don't mark port 0 removed -- we explicitly reserve it for
+     * backward compat with older guests, ensure a virtconsole device
+     * unplug retains the reservation.
+     */
+    if (port_id) {
+        unsigned int i;
+
+        i = port_id / 32;
+        vser->ports_map[i] &= ~(1U << (port_id % 32));
+    }
 
     port = find_port_by_id(vser, port_id);
     /*