virtio: reject shm region if length is zero

Prevent usage of shared memory regions where the length is zero,
as such configurations are not valid and may lead to unexpected behavior.

Signed-off-by: Sami Uddin <sami.md.ko@gmail.com>
Message-Id: <20250511222153.2332-1-sami.md.ko@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>