golang/xenlight: export keyed union interface types

golang/xenlight: export keyed union interface types

For structs that have a keyed union, e.g. DomainBuildInfo, the TypeUnion
field must be exported so that package users can get/set the fields
within. This means that users are aware of the existence of the
interface type used in those fields (see [1]), so it is awkward that the
interface itself is not exported. However, the single method within the
interface must remain unexported so that users cannot mistakenly "implement"
those interfaces.

Since there seems to be no reason to do otherwise, export the keyed
union interface types.

[1] https://pkg.go.dev/xenbits.xenproject.org/git-http/xen.git/tools/golang/xenlight?tab=doc#DeviceUsbdev

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>