softmmu/physmem: Never return directories from file_ram_open()

open() does not fail on directories when opening them readonly (O_RDONLY).

Currently, we succeed opening such directories and fail later during
mmap(), resulting in a misleading error message.

$ ./qemu-system-x86_64 \
    -object memory-backend-file,id=ram0,mem-path=tmp,readonly=true,size=1g
 qemu-system-x86_64: unable to map backing store for guest RAM: No such device

To identify directories and handle them accordingly in file_ram_open()
also when readonly=true was specified, detect if we just opened a directory
using fstat() instead. Then, fail file_ram_open() right away, similarly
to how we now fail if the file does not exist and we want to open the
file readonly.

With this change, we get a nicer error message:
 qemu-system-x86_64: can't open backing store tmp for guest RAM: Is a directory

Note that the only memory-backend-file will end up calling
memory_region_init_ram_from_file() -> qemu_ram_alloc_from_file() ->
file_ram_open().

Message-ID: <20230906120503.359863-8-david@redhat.com>
Reported-by: Thiner Logoer <logoerthiner1@163.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>