manifest_xml: use Superproject to hold XML content

Always create Superproject when there is a <superproject> tag, and have
it hold the XML content, similar to how other manifest elements are
handled.

This also adds SetQuiet and SetPrintMessages to Superproject
consistent with manifest.SetUseLocalManifests.

Change-Id: I522bf3da542006575799f0640c67f7052704f266
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334641


Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>