qdev-monitor: Fix check for full bus

Property bus has always been too screwed up to be really usable for
values other than plain bus IDs.  This just fixes a bug that crept in
in commit 1395af6f "qdev: add a maximum device allowed field for the
bus."

It doesn't always fail when it should:

    $ qemu-system-x86_64 -nodefaults -device virtio-serial-pci -device virtio-rng-device,bus=pci.0/virtio-serial-pci/virtio-bus

Happily plugs the virtio-rng-device into the virtio-bus provided by
virtio-serial-pci, even though its only slot is already occupied by a
virtio-serial-device.

And sometimes fails when it shouldn't:

    $ qemu-system-x86_64 -nodefaults -device virtio-serial-pci -device virtserialport,bus=virtio-bus/virtio-serial-device

Yes, the virtio-bus is full, but the virtio-serial-bus provided by
virtio-serial-device isn't, and that's the one we're trying to use.

Root cause: we check "bus full" when we resolve the first element of
the path.  That's the correct one only when it's also the last one.

Fix by moving the "bus full" check to right before we return a bus.

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