drop `-r` flag from `xargs` on OS X

OS X's `xargs` does not support the `-r` flag (which means "do not
execute the command even once if there are no arguments"), but
behaves by default as if it was specified.

You can verify this yourself with this test:

    $ touch zero-length-file
    $ xargs echo <zero-length-file

If `echo` is executed, you will see a blank line.  If it is not
executed (i.e. `-r` is specified or the behavior is implied) then
you will see no blank line.
Merge pull request #3902 from zigg/fix/osx-xargs-r-flag

guests/darwin: drop `-r` flag from `xargs` on OS X