qdev: Improve netdev property override error a bit

qdev: Improve netdev property override error a bit

qdev_prop_set_netdev() fails when the property already has a non-null
value.  Seems to go back to commit 30c367ed44
"qdev-properties-system.c: Allow vlan or netdev for -device, not
both", v1.7.0.  Board code doesn't expect failure, and crashes:

    $ qemu-system-x86_64 --nodefaults -nic user -netdev user,id=nic0 -global e1000.netdev=nic0
    Unexpected error in error_set_from_qdev_prop_error() at /work/armbru/qemu/hw/core/qdev-properties.c:1101:
    qemu-system-x86_64: Property 'e1000.netdev' doesn't take value '__org.qemu.nic0
    '
    Aborted (core dumped)

-device and device_add handle the failure:

    $ qemu-system-x86_64 -nodefaults -netdev user,id=net0 -netdev user,id=net1 -device e1000,netdev=net0,netdev=net1
    qemu-system-x86_64: -device e1000,netdev=net0,netdev=net1: Property 'e1000.netdev' doesn't take value 'net1'
    $ qemu-system-x86_64 -nodefaults -S -display none -monitor stdio -netdev user,id=net0 -netdev user,id=net1 -global e1000.netdev=net0
    QEMU 5.0.50 monitor - type 'help' for more information
    (qemu) qemu-system-x86_64: warning: netdev net0 has no peer
    qemu-system-x86_64: warning: netdev net1 has no peer
    device_add e1000,netdev=net1
    Error: Property 'e1000.netdev' doesn't take value 'net1'

Perhaps netdev property override could be made to work.  Perhaps it
should.  I'm not the right guy to figure this out.  What I can do is
improve the error message a bit:

    (qemu) device_add e1000,netdev=net1
    Error: -global e1000.netdev=... conflicts with netdev=net1

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-11-armbru@redhat.com>