scripts/add_maintainers.pl: Add logic to use V entry

scripts/add_maintainers.pl: Add logic to use V entry

Add logic to use V section entry in THE REST for identifying xen trees

Specifically:
* Move check until after the MAINTAINERS file has been read
* Add get_xen_maintainers_file_version() for check
* Remove top_of_tree as not needed any more
* Fail with extended error message when used out of tree

Signed-off-by: Lars Kurth <lars.kurth@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>