usb: Suppress bogus error when automatic usb-hub creation fails

USBDevice's realize method usb_qdev_realize() automatically creates a
usb-hub when only one port is left.  Creating devices in realize
methods is questionable, but works.

If usb-hub creation fails, an error is reported to stderr, but the
failure is otherwise ignored.  We then create the actual device using
the last port, which may well succeed.

Example:

    $ qemu -nodefaults -S -display none -machine usb=on -monitor stdio
    QEMU 2.2.50 monitor - type 'help' for more information
    (qemu) device_add usb-mouse
    [Repeat 36 times]
    (qemu) info usb
      Device 0.0, Port 1, Speed 12 Mb/s, Product QEMU USB Mouse
      Device 0.0, Port 2, Speed 12 Mb/s, Product QEMU USB Hub
      Device 0.0, Port 2.1, Speed 12 Mb/s, Product QEMU USB Mouse
    [More mice and hubs omitted...]
      Device 0.0, Port 2.8.8.8.8.7, Speed 12 Mb/s, Product QEMU USB Mouse
    (qemu) device_add usb-mouse
    usb hub chain too deep
    Failed to initialize USB device 'usb-hub'
    (qemu) info usb
    [...]
      Device 0.0, Port 2.8.8.8.8.7, Speed 12 Mb/s, Product QEMU USB Mouse
      Device 0.0, Port 2.8.8.8.8.8, Speed 12 Mb/s, Product QEMU USB Mouse

Despite the "Failed" message, the command actually succeeded.

In QMP, it's worse.  When adding the 37th mouse via QMP, the command
fails with

    {"error": {"class": "GenericError", "desc": "usb hub chain too deep"}}

Additionally, "Failed to initialize USB device 'usb-hub'" is reported
on stderr.  Despite the command failure, the device was created.  This
is wrong.

Fix by avoiding qdev_init() for usb-hub creation, so we can ignore
errors cleanly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
