qemu.tests.cfg: Disable "create -o" option related sub-cases

since qemu-img not support "create -o" option on rhel5 host,
so disable related sub-cases.

Signed-off-by: Shuping Cui <scui@redhat.com>
qemu.tests.qemu_img: Correct preallocated value

Preallocation mode (allowed values: off, metadata),
is not "no" or "yes".

Signed-off-by: Shuping Cui <scui@redhat.com>