virtio-serial-bus: Plug memory leak on realize() error paths

We neglect to free port->bh on the error paths.  Fix that.
Reproducer:
    {'execute': 'device_add', 'arguments': {'id': 'virtio_serial_pci0', 'driver': 'virtio-serial-pci', 'bus': 'pci.0', 'addr': '0x5'}, 'id': 'yVkZcGgV'}
    {'execute': 'device_add', 'arguments': {'id': 'port1', 'driver': 'virtserialport', 'name': 'port1', 'chardev': 'channel1', 'bus': 'virtio_serial_pci0.0', 'nr': 1}, 'id': '3dXdUgJA'}
    {'execute': 'device_add', 'arguments': {'id': 'port2', 'driver': 'virtserialport', 'name': 'port2', 'chardev': 'channel2', 'bus': 'virtio_serial_pci0.0', 'nr': 1}, 'id': 'qLzcCkob'}
    {'execute': 'device_add', 'arguments': {'id': 'port2', 'driver': 'virtserialport', 'name': 'port2', 'chardev': 'channel2', 'bus': 'virtio_serial_pci0.0', 'nr': 2}, 'id': 'qLzcCkob'}

The leak stack:
Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x7f04a8008ae8 in __interceptor_malloc (/lib64/libasan.so.5+0xefae8)
    #1 0x7f04a73cf1d5 in g_malloc (/lib64/libglib-2.0.so.0+0x531d5)
    #2 0x56273eaee484 in aio_bh_new /mnt/sdb/backup/qemu/util/async.c:125
    #3 0x56273eafe9a8 in qemu_bh_new /mnt/sdb/backup/qemu/util/main-loop.c:532
    #4 0x56273d52e62e in virtser_port_device_realize /mnt/sdb/backup/qemu/hw/char/virtio-serial-bus.c:946
    qemu#5 0x56273dcc5040 in device_set_realized /mnt/sdb/backup/qemu/hw/core/qdev.c:891
    qemu#6 0x56273e5ebbce in property_set_bool /mnt/sdb/backup/qemu/qom/object.c:2238
    qemu#7 0x56273e5e5a9c in object_property_set /mnt/sdb/backup/qemu/qom/object.c:1324
    qemu#8 0x56273e5ef5f8 in object_property_set_qobject /mnt/sdb/backup/qemu/qom/qom-qobject.c:26
    qemu#9 0x56273e5e5e6a in object_property_set_bool /mnt/sdb/backup/qemu/qom/object.c:1390
    qemu#10 0x56273daa40de in qdev_device_add /mnt/sdb/backup/qemu/qdev-monitor.c:680
    qemu#11 0x56273daa53e9 in qmp_device_add /mnt/sdb/backup/qemu/qdev-monitor.c:805

Fixes: 199646d
Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Amit Shah <amit@kernel.org>
Message-Id: <20200309021738.30072-1-pannengyuan@huawei.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: panny060

hw/char/virtio-serial-bus.c

@@ -943,7 +943,6 @@ static void virtser_port_device_realize(DeviceState *dev, Error **errp)
     Error *err = NULL;
 
     port->vser = bus->vser;
-    port->bh = qemu_bh_new(flush_queued_data_bh, port);
 
     assert(vsc->have_data);
 
@@ -992,6 +991,7 @@ static void virtser_port_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    port->bh = qemu_bh_new(flush_queued_data_bh, port);
     port->elem = NULL;
 }