virtio_pci: Use device_probe_and_attach(), instead of DEVICE_ATTACH().

* By directly calling DEVICE_ATTACH(), we didn't get automatically created
  sysctl nodes (added in 8e4d13764bdfc916c23fb22158c65e486dce631c), and
  we didn't get automatic dmesg messages on child driver attachement
  (e.g. "vtblk0: <VirtIO Block Adapter> on virtio_pci1") .

* Since we have no direct equivalent to FreeBSD's device_attach(), use
  device_probe_and_attach(). This means we do a vtpci_set_status() cycle
  even if no matching driver is found.