The `pkg_add` command will return `0` when a package requested for
installation is not found. This adds a validation check to ensure
the rsync package is actually installed on the guest.
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.