docs: Document configuration quirks for virtio devices
docs: Document configuration quirks for virtio devices

Some devices (namely virtio-scsi, virtio-gpu, virtio-keyboard,
virtio-tablet and virtio-mouse, plus virtio-crypto which is
not supported by libvirt) don't follow the same rules as all
other virtio devices, which is something that ought to be
documented.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Cole Robinson <7cd45ff6ad1fb7cf556a02797d774247851499f8@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,jfehlig/libvirt,zippy2/libvirt,eskultety/libvirt,zippy2/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,fabianfreyer/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,fabianfreyer/libvirt