qdev-monitor: Avoid device_add crashing on non-device driver name

Watch this:

    $ upstream-qemu -nodefaults -S -display none -monitor stdio
    QEMU 1.7.50 monitor - type 'help' for more information
    (qemu) device_add rng-egd
    /work/armbru/qemu/qdev-monitor.c:491:qdev_device_add: Object 0x2089b00 is not an instance of type device
    Aborted (core dumped)

Crashes because "rng-egd" exists, but isn't a subtype of TYPE_DEVICE.
Broken in commit 18b6dad.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
