usb: Do not prefix error_setg() messages with "Error: "

Because it produces beauties like

    (qemu) usb_add mouse
    Failed to initialize USB device 'usb-mouse': Error: tried to attach usb device QEMU USB Mouse to a bus with no free ports

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