softmmu/physmem: Bail out early in ram_block_discard_range() with readonly files

fallocate() will fail, let's print a nicer error message.

Message-ID: <20230906120503.359863-6-david@redhat.com>
Suggested-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>