hostmem/shm: Allow shm memory backend serve as shared memory for coco-VMs

hostmem/shm: Allow shm memory backend serve as shared memory for coco-VMs

shm can surely serve as the shared memory for coco-VMs. But currently it
doesn't check the backend->guest_memfd to pass down the RAM_GUEST_MEMFD
flag. It leads to failure when creating coco-VMs (e.g., TDX guest) which
require private mmeory.

Set and pass down RAM_GUEST_MEMFD when backend->guest_memfd is true, to
allow shm memory backend serve as shared memory for coco-VMs.

Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20250721065220.895606-1-xiaoyao.li@intel.com
Signed-off-by: Peter Xu <peterx@redhat.com>