virtio_net_device_realize() rejects invalid duplex and speed values.
The error handling is broken:

    $ ../qemu/bld-sani/x86_64-softmmu/qemu-system-x86_64 -S -display none -monitor stdio
    QEMU 4.2.93 monitor - type 'help' for more information
    (qemu) device_add virtio-net,duplex=x
    Error: 'duplex' must be 'half' or 'full'
    (qemu) c
    =================================================================
    ==15654==ERROR: AddressSanitizer: heap-use-after-free on address 0x62e000014590 at pc 0x560b75c8dc13 bp 0x7fffdf1a6950 sp 0x7fffdf1a6940
    READ of size 8 at 0x62e000014590 thread T0
	#0 0x560b75c8dc12 in object_dynamic_cast_assert /work/armbru/qemu/qom/object.c:826
	#1 0x560b74c38ac0 in virtio_vmstate_change /work/armbru/qemu/hw/virtio/virtio.c:3210
	#2 0x560b74d9765e in vm_state_notify /work/armbru/qemu/softmmu/vl.c:1271
	#3 0x560b7494ba72 in vm_prepare_start /work/armbru/qemu/cpus.c:2156
	#4 0x560b7494bacd in vm_start /work/armbru/qemu/cpus.c:2162
	#5 0x560b75a7d890 in qmp_cont /work/armbru/qemu/monitor/qmp-cmds.c:160
	#6 0x560b75a8d70a in hmp_cont /work/armbru/qemu/monitor/hmp-cmds.c:1043
	#7 0x560b75a799f2 in handle_hmp_command /work/armbru/qemu/monitor/hmp.c:1082
    [...]

    0x62e000014590 is located 33168 bytes inside of 42288-byte region [0x62e00000c400,0x62e000016930)
    freed by thread T1 here:
	#0 0x7feadd39491f in __interceptor_free (/lib64/libasan.so.5+0x10d91f)
	#1 0x7feadcebcd7c in g_free (/lib64/libglib-2.0.so.0+0x55d7c)
	#2 0x560b75c8fd40 in object_unref /work/armbru/qemu/qom/object.c:1128
	#3 0x560b7498a625 in memory_region_unref /work/armbru/qemu/memory.c:1762
	#4 0x560b74999fa4 in do_address_space_destroy /work/armbru/qemu/memory.c:2788
	#5 0x560b762362fc in call_rcu_thread /work/armbru/qemu/util/rcu.c:283
	#6 0x560b761c8884 in qemu_thread_start /work/armbru/qemu/util/qemu-thread-posix.c:519
	#7 0x7fead9be34bf in start_thread (/lib64/libpthread.so.0+0x84bf)

    previously allocated by thread T0 here:
	#0 0x7feadd394d18 in __interceptor_malloc (/lib64/libasan.so.5+0x10dd18)
	#1 0x7feadcebcc88 in g_malloc (/lib64/libglib-2.0.so.0+0x55c88)
	#2 0x560b75c8cf8a in object_new /work/armbru/qemu/qom/object.c:699
	#3 0x560b75010ad9 in qdev_device_add /work/armbru/qemu/qdev-monitor.c:654
	#4 0x560b750120c2 in qmp_device_add /work/armbru/qemu/qdev-monitor.c:805
	#5 0x560b75012c1b in hmp_device_add /work/armbru/qemu/qdev-monitor.c:905
    [...]
    ==15654==ABORTING

Cause: virtio_net_device_realize() neglects to bail out after setting
the error.  Fix that.

Fixes: 9473939e
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422130719.28225-9-armbru@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>