golang/xenlight: generate structs from the IDL

golang/xenlight: generate structs from the IDL

Add struct and keyed union generation to gengotypes.py. For keyed unions,
use a method similar to gRPC's oneof to interpret C unions as Go types.
Meaning, for a given struct with a union field, generate a struct for
each sub-struct defined in the union. Then, define an interface of one
method which is implemented by each of the defined sub-structs. For
example:

  type domainBuildInfoTypeUnion interface {
          isdomainBuildInfoTypeUnion()
  }

  type DomainBuildInfoTypeUnionHvm struct {
      // HVM-specific fields...
  }

  func (x DomainBuildInfoTypeUnionHvm) isdomainBuildInfoTypeUnion() {}

  type DomainBuildInfoTypeUnionPv struct {
      // PV-specific fields...
  }

  func (x DomainBuildInfoTypeUnionPv) isdomainBuildInfoTypeUnion() {}

  type DomainBuildInfoTypeUnionPvh struct {
      // PVH-specific fields...
  }

  func (x DomainBuildInfoTypeUnionPvh) isdomainBuildInfoTypeUnion() {}

Then, remove existing struct definitions in xenlight.go that conflict
with the generated types, and modify existing marshaling functions to
align with the new type definitions. Notably, drop "time" package since
fields of type time.Duration are now of type uint64.

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