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>
qemu.tests.qemu_img: Correct create command with -o option

if preallocated == metadata and cluster_size is not None, will get command like:
qemu-img create -f qcow2 -o prellaction=metadata -o cluster_size=xxx,
correct command should be: qemu-img create -f qcow2 -o prellaction=metadata,cluster_size=xxx

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #16 from spcui/qemu_img_fix

qemu.tests: qemu_img.py bugs fix