virtio: assert on PCI configuration failure

in the virtio constructor we need to assert that the PCI config
parsing is successful.
Found while developing / integrating a new generic QEMU PCI host
implementation for the virt platform.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>