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.