virtfs: Fix documentation of -fsdev and -virtfs

This fixes several things:
- add "id" description to -virtfs documentation
- split the description into several lines in both usage and documentation
  for accurateness and clarity
- add documentation and usage of the synth fsdriver
- add "throttling.*" description to -fsdev local
- add some missing periods
- add proper reference to the virtfs-proxy-helper(1) manual page
- document that the virtio device may be either virtio-9p-pci, virtio-9p-ccw
  or virtio-9p-device, depending on the machine type

Buglink: https://bugs.launchpad.net/qemu/+bug/1581976


Signed-off-by: Greg Kurz <groug@kaod.org>
Acked-by: Thomas Huth <thuth@redhat.com>