tests: fix virtio-blk-test leaks

Use qvirtio_pci_device_find_slot() to avoid leaking the non-hp
device. Add assert() to avoid further leaks in the future.

Use qvirtio_pci_device_free() to correctly free QVirtioPCIDevice.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
