virtio/virtio_pci_legacy_dev: ensure the correct return value

When pci_iomap return NULL, the return value is zero.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Link: https://lore.kernel.org/r/20211222112014.87394-1-flyingpeng@tencent.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>