combogrid: add handling for historic set values currently not available

combogrid: add handling for historic set values currently not available

We can often run into situations where a value set in the past is not
valid anymore. An example could be a deleted network bridge, e.g., we
set the vNIC of a VM to 'vmbr1' but then we decide to obsolete that
and delete that one, one would now expect that the field gets marked
as invalid when editing the VM's vNIC, so add that behavior.

As sometimes this can be valid and wanted behavior (e.g., usb
passthrough, which is hot-pluggable), also add a switch do restore
the old behavior.

Note that the empty value is not handled by this change, we let the
existing "allowBlank" config switch handle that one.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>