qemu.tests: make qemu-img covert more test scenario

Test plan updated, we need to make qemu_img test option 'preallation'
and 'cluster_size' togethor to avoid bug like:
qemu-img coredump when create image with cluster_size > 65536 and
option 'preallation=metadata';

Signed-off-by: Xu Tian <xutian@redhat.com>