virtio: use modern mode for transitional device by default

[timo.voelker_fh-muenster.de]

Summary

This patch changes the default value of the loader tunable hw.virtio.pci.transitional to 1. This means, virtio uses the modern mode for transitional devices by default.

Also, this patch corrects the description of the loader tunable in the man page.

[timo.voelker_fh-muenster.de]

Why changing hw.virtio.pci.transitional to 1 by default:

• VirtualBox provides a transitional VirtIO network device, which seems not to work in legacy mode. By using modern mode by default, FreeBSD does not run in this VirtualBox bug (by default).
• The return values of vtpci_modern_probe and vtpci_legacy_probe were chosen to prefer modern mode but hw.virtio.pci.transitional=0 prevents modern mode. Setting hw.virtio.pci.transitional to 1 by default seems a better fit.