migration/savevm: Prepare vmdesc json writer in qemu_savevm_state_setup()

migration/savevm: Prepare vmdesc json writer in qemu_savevm_state_setup()

... and store it in the migration state. This is a preparation for
storing selected vmds's already in qemu_savevm_state_setup().

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>