tests: Avoid xargs, for FreeBSD compatibility.

The FreeBSD version of xargs does not run the utility argument on empty
input, while GNU xargs runs it at least once, even with empty input.  As
a result on FreeBSD VSCTL_CHECK_FIND returned no output for an empty
bridge list while on Linux it returned a single blank line.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>