Creating a model with a single valued optional referential value panics.

[alexanderConstantinescu]

.Create() currently panics if the model created specifies a single valued optional referential value, ex: create a logical router static route as:

 nbdb.LogicalRouterStaticRoute{
		UUID:   logicalRouterStaticRouteNamedUUID,
		Policy: &nbdb.LogicalRouterStaticRoutePolicySrcIP,
                BFD: &myBFDUUID, 
	}

It will panic with the following stack trace:

panic: interface conversion: interface {} is *string, not []string

goroutine 1429 [running]:
github.com/ovn-org/libovsdb/ovsdb.NativeToOvs(0xc00a99b230, 0x24aa1e0, 0xc006366160, 0x24aa100, 0xc006366160, 0x0, 0x0)
        /home/aconstan/go/src/github.com/ovn-org/ovn-kubernetes/go-controller/vendor/github.com/ovn-org/libovsdb/ovsdb/bindings.go:257 +0x1147
github.com/ovn-org/libovsdb/mapper.Mapper.NewRow(0xc00cc4f380, 0x27bb8c0, 0x1b, 0x248bca0, 0xc009a04fc0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/aconstan/go/src/github.com/ovn-org/ovn-kubernetes/go-controller/vendor/github.com/ovn-org/libovsdb/mapper/mapper.go:155 +0x614
github.com/ovn-org/libovsdb/client.api.Create(0xc00634bb30, 0x0, 0x0, 0xc0063662e0, 0x1, 0x1, 0x10, 0xc00f90f060, 0x44164b, 0x418980, ...)
        /home/aconstan/go/src/github.com/ovn-org/ovn-kubernetes/go-controller/vendor/github.com/ovn-org/libovsdb/client/api.go:254 +0x466
github.com/ovn-org/libovsdb/client.(*ovsdbClient).Create(0xc00f44db80, 0xc0063662e0, 0x1, 0x1, 0x248bca0, 0xc009a04fc0, 0x27cb71f, 0x25, 0xc0063662d0)
        /home/aconstan/go/src/github.com/ovn-org/ovn-kubernetes/go-controller/vendor/github.com/ovn-org/libovsdb/client/client.go:592 +0x90

It since LogicalRouterStaticRoute.BFD is optional, it will be parsed as a set https://github.com/ovn-org/libovsdb/blob/main/ovsdb/schema.go#L531, this is however not handled properly it seems in bindings (which always assumes a slice of referential values): https://github.com/ovn-org/libovsdb/blob/main/ovsdb/bindings.go#L257

[flavio-fernandes]

@dave-tucker : Is it proper to assume that this issue introduced by #204 ?