physmem: Replace check for RAMBlock offset 0 with xen_mr_is_memory

physmem: Replace check for RAMBlock offset 0 with xen_mr_is_memory

For xen, when checking for the first RAM (xen_memory), use
xen_mr_is_memory() rather than checking for a RAMBlock with
offset 0.

All Xen machines create xen_memory first so this has no
functional change for existing machines.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-ID: <20240529140739.1387692-6-edgar.iglesias@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>