qemu.tests.cfg: Disable pre_cmd and post_cmd for RHEL3, 4, 5, 6 guest

There is no /usr/lib/udev/rules.d in these guest.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #225 from spcui/cdrom_test_fix

qemu.tests.cfg: Disable pre_cmd and post_cmd for RHEL3, 4, 5, 6 guest