qdev-monitor: Stop error avalanche in qbus_find_recursive()

Reproducer:

    $ qemu-system-x86_64 -nodefaults -device virtio-rng-pci -device virtio-rng-pci -device virtio-rng-device,bus=virtio-bus
    qemu-system-x86_64: -device virtio-rng-device,bus=virtio-bus: Bus 'virtio-bus' is full
    qemu-system-x86_64: -device virtio-rng-device,bus=virtio-bus: Bus 'virtio-bus' is full
    qemu-system-x86_64: -device virtio-rng-device,bus=virtio-bus: Bus 'virtio-bus' not found

qbus_find_recursive() reports the "is full" error itself, and leaves
reporting "not found" to its caller.  The result is confusion.  Write
it a function contract that permits leaving all error reporting to the
caller, and implement it.  Update callers to detect and report "is
full".

Screwed up when commit 1395af6 added the max_dev limit and the "is
full" error condition to enforce it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
