We need Device IDs to be unique and not contain '/' so device tree
nodes can always be unambigously referenced by tree path.
We already have some protection against duplicate IDs, but it got
holes:
* We don't assign IDs to default devices.
* -device and device_add use the ID of a qemu_device_opts. Which
rejects duplicate IDs.
* pci_add nic -net use either the ID or option "name" of
qemu_net_opts. And there's our hole. Reproducible with "-net user
-net nic,id=foo -device lsi,id=foo".
Also require IDs to start with a letter to provide for possible future
extensions.