synced_folders/rsync: only chown when necessary

Run remote rsync as root to guarantee that rsync can write to guestpath.
This obviates the need to chown the guestpath to the SSH user prior to
sync.

This brings a substantial speedup (2x on a moderately-sized shared
folder) and properly triggers filesystem notifications on only the files
changed by a given sync.

This is the fix I described for #3525—I figured I'd put my money where my mouth is.

Tested with an Ubuntu 12.04 box and works like a charm.

Note that though I changed the capabilities for all guests that support rsync, I haven't actually tested on those OSes. I figured I'd wait for your feedback before trying to test this everywhere.