virttest.utils_selinux: Make it easier to locate SELinux commands

In older distros, such as RHEL5, /usr/sbin is outside of
PATH, so trying to find out SELinux utility commands will
fail. Use utils_misc.find_command to fix that and make
unittests to pass on RHEL5.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>