shard.cfg.guest-os: Update config file sturcture for RHEL

This patch adds an extra directory level for RHEL guest,
so that we can add arch independent config in single place.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>

I tested this patch with a little shell script.
```
for g in RHEL.3.9 RHEL.4.7 RHEL.4.8 RHEL.5.3 RHEL.5.4 RHEL.5.5 RHEL.5.6 RHEL.5.7 RHEL.5.8 RHEL.5.9 RHEL.6.0 RHEL.6.1 RHEL.6.2 RHEL.6.3 RHEL.6.4 RHEL.6.devel; do
        for arch in i386 x86_64; do
                git checkout next > /dev/null 2>&1;
                ./qemu/update_config.py > /dev/null 2>&1;
                guest=$g.$arch
                echo "include qemu/cfg/tests.cfg" > t.cfg;
                echo "only $guest" >> t.cfg;
                echo "only unattended_install" >> t.cfg;
                echo -n "$guest "
                virttest/cartesian_config.py --content t.cfg > /tmp/before.$guest;
                git checkout linux-cfg > /dev/null 2>&1;
                ./qemu/update_config.py > /dev/null 2>&1;
                virttest/cartesian_config.py --content t.cfg > /tmp/after.$guest;
                diff -u /tmp/before.$guest /tmp/after.$guest || echo -n " has problem."
                echo -ne "\n"
        done
done
```
These guests' config looks work well, except RHEL.6.1.x86_64 and RHEL.6.devel.i386, because:
 * We don't have config for RHEL.6.1.x86_64 before.
 * The RHEL.6.devel.i386 has some know bug which will be fixed in this patch: it doesn't have vm_arch_name parameter and the image_name is in incorrect config block.
```
- 6.devel.i386:
    no setup
    nic_hotplug:
        modprobe_module =
    block_hotplug:
        modprobe_module =
        image_name = images/rhel6devel-32
```