virtio: pci: Check virtio configs are mapped

Prepare for calls to `virtio_pci_map_capability()` failing by returning
NULL on error. If this happens, later accesses to the pointers would be
unsafe so cause the probe to fail if such an error occurs.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>