physmem: factor out memory_region_is_ram_device() check in memory_access_is_direct()

physmem: factor out memory_region_is_ram_device() check in memory_access_is_direct()

As documented in commit 4a2e242bbb306 ("memory: Don't use memcpy for
ram_device regions"), we disallow direct access to RAM DEVICE regions.

Let's make this clearer to prepare for further changes. Note that romd
regions will never be RAM DEVICE at the same time.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250210084648.33798-2-david@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>